I think it might be more dramatic if the creatures ignore the lich, but drag all his lich's treasures from the tower, and as he gives chase, he starts to find his things, one at a time, abandoned along the trail. The first few items, he doesn't even notice what's missing, but then he finds a scroll perhaps, with a square torn out of it. One of the creatures has ripped out a bit of the illumination — nothing of actual arcane significance, you understand, just a brightly colored design; he can't even imagine what they'd want it for. Only then does he recognize that ribbons, bookmarks, gold leaf, and such like are missing from all the items. If he's forgotten what beauty is, he may be utterly confused, and completely unable to figure out the pattern. If he spends some time trying to work it out, that could allow you to hint to the reader about what has happened without the character necessarily realizing it.
"Ah, look, the Scroll of Bob! The creatures have unrolled it, but it looks undamaged. I guess they couldn't read and lost interest. What fools they were! They beheld the great secrets of Bob, but saw no value in them. I will tie it closed — but wait, the red silken cord woven by Bob himself is missing. I wonder where it is, ah, no matter, it had no real significance. I can tie the scroll with any old piece of twine."
"Hm, the Book of Jim which I found earlier is also damaged. I didn't notice before, but the golden replica of the 5th Seal has been pried off the cover. Yet, the symbol is reproduced more accurately on page 45. Those morons stole the wrong seal! Ha, what inept thieves they were."
…and such.
Or if you wanted to imply that the swarm of creatures were agents of the Magpies, then all the stolen items could be found piled haphazardly in a clearing in the forest. The swarm has delivered everything to their masters, who then picked and chose what they wanted, and got away before the lich arrived.
(account deleted) 05 Nov 2016 16:07