Have you been able to correlate any of the appearances with specific things your group is working on? Times matching up, etc.? I'm guessing these things aren't coming back in their last state of destruction or there wouldn't be a question of if they're really from the past or not?
(And you can't put conundrums out there like that without my wanting to ask questions and poke into it. You know that, right?)
I wonder if there are any spells for the magical equivalent of carbon dating?