In the third chapter of Battleground, Harry says that an Outsider recognized and said what Mac was.
On page222 [hardback] of Cold Days Sharkface says to Mac
"You have no place in this, watcher.Do you think this gesture has meaning? It is every bit as empty as you. You chose your road long ago. Have the grace to lie down and die beside it.
Then I looked up the term and found this in Wikipedia;
Watcher (Aramaic עִיר ʿiyr, plural עִירִין ʿiyrin, [ʕiːr(iːn)]; Theodotian trans: ir; from the root of Heb. ʿer, "awake, watchful".[1] Greek: ἐγρήγοροι, transl.: egrḗgoroi; "Watchers", "those who are awake"; "guard", "watcher"[2]) is a term used in connection with biblical angels. Watcher occurs in both plural and singular forms in the Book of Daniel (4th–2nd century BC), where reference is made to their holiness. The apocryphal Books of Enoch (2nd–1st centuries BC) refer to both good and bad Watchers, with a primary focus on the rebellious ones.[3][4]
This fits Mac more specifically, this watcher;
Shamsiel, once a guardian of Eden as stated in the Zohar, served as one of the two chief aides to the archangel Uriel (the other aide being Hasdiel) when Uriel bore his standard into battle, and is the head of 365 legions of angels and also crowns prayers, accompanying them to the 5th heaven. In Jubilees, he is referred to as one of the Watchers. He is a fallen angel who teaches the signs of the sun.
Connected to Uriel, Mac is a fallen angel named Shamsiel. Yes, he'd recognize the Placard when he saw it. Yes, looking upon him with his wizard's sight would indeed blind or worse Harry. Yes, Mac would know when Harry stopped using it.
Further is says about watchers;
The watchers are bound "in the valleys of the Earth" until Judgment Day (Jude verse 6 says, "And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.").
This fits Mac, he is bound to Earth but for the most part stays out of almost everything, but he might be hoping for redemption now, he committed himself with his blood on the Placard to sacrifice himself if necessary for those in his charge.