I think that line is ambiguous at best, Harry says, but it was pretty clear that he didn’t like that I’d realized what he knew.

Because the line before that Harry says to Mac, "You know what this is."I said.

That wasn't a question, it was a declarative sentence, then Harry says, " he didn’t like that I’d realized what he knew."

Harry uses "what" not who, Mac didn't like that fact that Harry knew that he knew "what" that old piece of wood was.

I think we're back to ATD, as we're interpreting the text differently.  ¯\_(ツ)_/¯

Or it has to be triggered by an event perhaps?  Because if Justine wasn't infected before Cold Days it had to have happened at some point since. 

In Blood Rites it could have happened.  HWWBh emerges from Madge's body.

The thing is, it is something Harry remembers, but he doesn't say anything to HWWBh, nothing happens after that.  Madge's body deflates, and poof, the Outsider is gone?  How odd is that there is no direct interaction between an Outsider and Harry? Not one wise ass remark on Harry's part.  It is also a very weak Justine who directed Harry and company to go and save Thomas.  Was that what she was doing?  Or was her real motive to get Harry killed trying to save Thomas?  Long and the short of it not totally out of the question that she was infected, heck it might be the reason she survived the heavy feeding from the badly injured Thomas.

I'd still rather we have an event where she was at least exposed to Beside. 

For now, I'd settle for something like this.
Stages of Beside's Influence

• Contraction - A victim is exposed to Outside power, staining them.
• Impulse - The stain grows, subtly changing their nature without direct involvement from Beside or anyone else.
• Influence - Beside can begin to whisper to them, as a voice inside their head, more directly affecting their decision making.
• Cooperation - The victim becomes aware of Beside's presence, and agrees to work with him for mutual gain.
• Domination - The victim has lost control to Beside, to the point that he can possess them and control their every action.

That would be the natural order of operations for mortals, but an object like the Athame can expedite the infection to the point of near-immediate domination.

And we could see different levels of 'infection' in different victims. (Most of these are hypothetical)

• Sells - Impulse. His nature changed, but Beside never possessed him.
• Denton - Impulse. The belt changed his nature, but Beside never got involved.
• Kravos - Influence. Guy might very well have heard voices that progressed things along.
• Aurora - Influence. Beside whispered to her, influencing her ideas and plans, but without her awareness.
• Book 5? Mavra?
• Book 6? Lord Raith?
• Cowl - Influence. Maybe he's hearing voices, or maybe he's already on to cooperation.
• Lea - Domination. Thanks to the Athame, her progression was swift, but then suppressed by Mab.
• Vitto - Cooperation. He made deals for power to the point he was 'possessed' (most likely hosted) an Outsider. (Per weak WoJ, possibly Before)
• Namshiel's host - Cooperation. Possibly against the will of the Fallen.
• Book 11? Madeline? Skinwalker?
• Arianna - Cooperation? Trying to usurp her father?
• Book 13?
• Maeve - Cooperation. She was never controlled by Beside, but after becoming aware, she became a
• Cat Sith - Domination. Maeve cut corners and quickly advanced things, due to her authority over Sith and his lack of free will.

willing conspirator.[/li]
[li] Book 15?[/li]
[li] Justine - Domination by then, but when did it begin?[/li]
[/list]

Potential sources for Justine's infection.

• Grave Peril - Mavra, Bianca, or the Nightmare infected her after the party. It started slow and ramped up over the years.
• Turn Coat - Skinwalker infected her with his belly swipe attack while attacking Raith manor. By Cold Days she knew she was infected but not cooperating, a la Harry and Lash. Maeve wanted to hurry things along at the end of the book.
• Even Hand - Justine was exposed to something on the mission to the Fomor territory just before the short story events, and Beside was already beginning to control.

Not with the placard. An outsider couldn't get inside. That is what I understood I mean. An outsider couldn't enter the place unless someone killed Mac first.

We don't know what the placard does, specifically, other than protect a place.  We don't know if it would do anything other than reinforce the threshold, or if it would do something to stop mortals with ill intent as well. 

If Justine had walked in there, it might have let her in, if she did so under her own power and without ill intent.

It was first mentioned in Death Masks.

I stand corrected.

And if it is neutral territory, everyone is welcome, including the Fomor, not to tear up the other patrons, but welcome because someone neutral doesn't take sides... Thus by smearing his blood on the Placard and hanging it up, Mac took a side against the Fomor.. They weren't getting in his place.

The placard was set to defend the place and make Mac the conduit or link to the place. We don't know anything about how it works, but if someone showed up looking for shelter, including the Fomor, it might have let them in. We don't know the rules of how it works.

So saying he took sides to me is an overstep.  He just did more of what he was already going to do, but at severely higher risk to himself.

It was all about the Placard and the meaning of it that Mac was reacting to.  Not Harry figuring out what he is, Mac wasn't upset,...

I don't know why you're ignoring the text. Here's the direct quote.

“You know what it is,” I said.
Mac rocked back half a pace. He looked from the sign to me. He didn’t quite lick his lips in nervous guilt, but it was pretty clear that he didn’t like that I’d realized what he knew.

The text says it's Harry's knowledge that upsets him, not the placard.

Wasn't he already doing that when Harry brought in the Placard.  But hold it a minute, Mac's Bar is neutral territory, different factions meet there from time to time.  Did Mac ever sign the Accords? If he had, he cannot reside over neutral territory, signing the Accords is taking a side.

He's had a sign on the wall since the first book that says it's Accorded Neutral Territory.  He filled out the necessary paperwork.

He may not be a member of the Accorded Nations, but he's recognized as a neutral party, and his place is a neutral location. And I'd bet money that he's obligated to recognize the rules about guest law and whatnot.

This is dragging up an older topic but I wanted to share my thoughts.
I suspect Justine was infected around the time of White Night (likely after but possibly before). It was right around the time she was clearly becoming useful to Lara and Nemesis could have learned of her through Vittorio either during or before the Deeps. The Justine we saw there was much more functional than the one we'd seen before (though meds were her given reason) but the Justine we saw from every other interaction was much more functional than that. Its likely she accepted Nemisis willingly (without it revealing what it really was) under the promise that it could reunite her with Thomas. (Which it did...) After that she was likely manipulating things from the background. Like pitting Marcone vs Formor in Even Hand.

I think it depends on the nature of possession by Beside, which we don't know for sure. I could see it being a quick forced possession in the Even Hand adventure, or I could see it being a slow seduction possession starting around White Night.

But the comment by Maeve in Cold Days just seems damning to the latter. If Justine was already possessed and in Beside's control, there'd be no need for Maeve to call out that Justine was of interest to her, being so close to Lara. 

Even if Justine was being slowly corrupted at that point, it seems unlikely that Maeve would spell out the reason that she was already being corrupted.  It seems much more likely to me that Maeve and Beside had genuine interest in getting to Justine, and that slipped out when Maeve thought her win was assured.

Exactly, but putting his blood on the Placard and hanging it up, is.

That's your opinion.

But Mac would do that anyway, he'd stand there with his shotgun.

Yes. Which isn't taking sides.