... Honestly, if he really cared, couldn't he just get exactly how many years ago merlin was on the island (each time) and for how long, etc?
It could mean that intellectus just covers the present state of the island. That is, Harry knows where the wasp nest is now but not the location of the previous nest.
Interesting ideas... very!
Harry has to actively engage with the topic. If he is running, he knows what's underfoot, what's on his route (considering a goal, he knows the best route); if he wonders what animals are on the island, or plants, or etc etc etc... he knows any of that. But he doesn't "know" any topic that he doesn't actively consider, actively interact with.
So...
Has Harry actively considered the island's history, while on the island? I have to presume that he
did indeed do so in the long interval between Cold Days and Skin Game, but... that was offscreen, and we don't know what (if any) results he got. Damn that Butcher guy...
Do we have ANY onscreen instances of Harry-the-Warden-of-Demonreach trying to use intellectus to consider anything historical about the island? Or explicitly querying Alfred? Even mundane stuff, like the last time more than a few people overnight'ed there (prohibition smugglers, I presume (oh, hey -- d'you suppose there's some
amazing aged Canadian whiskey buried somewhere on Demonreach??? Harry could probably score serious points with Mac, if he brought a case of that...
And obviously, anything like that buried there now, Harry just has to wonder about, to know where it is... ).
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But I'm pretty sure that all things Magical and Merlin-centric are explicitly NOT covered by the intellectus effect:
- As I noted above, Harry wasn't sure if he could cross the empowered circle, near the end of Cold Days.
- As others noted above, Harry needed Bob to "translate" the wibbly-wabbly, magico-wagico stuff that Alfred actively wanted Harry to know.
- Harry has long wondered about the runes &c on the island, and would certainly have speculated (and learned whatever the intellectus could tell him) about them, during the CD/SG interval.
- In Turn Coat, Harry sets up a fight on the island to unmask the Bad Guy; Binder is one of the Bad Guy's hired hands, and Harry knows when each of the sumoned goons steps onto the island, but only infers the summoning-circle; he doesn't "know" it (<insert PSY/Gagnam voice> intellectus-style).
I suspect this is intentional: the magic is described as more complex than anything Bob can even grasp; a direct grasp of this thrust into Harry's brain might break him. I assume Merlin built the intellectus effect intentionally as "covers nothing magical."
This still leaves open the question of whether the intellectus knows anything other than current state of the island, i.e. mundane history.