Thanks for your replies. I discussed it with a friend of mine this evening, and that helped. I needed a little time to chill out, I think! I plan to have a discussion with him at a relaxed moment sometime in the next couple days and just convey that I see he's having a hard time controlling his impulses and suggest that we brainstorm some solutions to try. (He already suggested sewing his pockets shut!) Maybe having a couple of his own toys in his pockets will keep him from wanting to fill them.
Yes, he already returned the things. I wasn't at the latest meeting, so I'm not really sure if he apologized. I was thinking maybe I'd suggest that he write a brief apology to his teacher.
The only "trigger" I can really point to is that he's just in the past month become hyper aware of having money and what it can buy. So maybe he's just feeling particularly acquisitive. The other thing is that he's quite a scavenger for "junk" he finds - rubber bands, ribbons, feathers, bottlecaps, etc - and tends to load his pockets up with them. I hear "I could use this in an art project!" several times a day. (I've given up on the cleanliness issue, though I draw the line at used popsicle sticks at the park.) I'm thinking at school, this place with 325 kids who often drop unwanted items, that he must be sort of on "scavenge alert" all the time. Maybe the alert got set too high somehow and now he's starting to want to scavenge useful things that actually belong to people. Now that i think of it, it's probably difficult to differentiate that fine line between, "sure, you can take that rock from the parking lot" and "no, you can't take things from your classroom!"
His teacher was pretty cool and said, "if you want to borrow something from the classroom overnight that might be OK, but I do need you to ask me first."
Deeporgarten, reading your comments about the development of empathy was really helpful. Thank you - this is just what I need to hear.
Lilyka, I've been wondering if it's something that's going on at school, but I think your latter suggestion is correct - it's just where he spends most of his away-from-home time. And the buttons *were* lovely - apparently there was quite a furor of button-craft activity the first day during their choice time, which is coincidentally the day ds missed out on choice time because he was finishing up other work.

So I'm thinking he felt like he missed out on the cool new stuff and decided to take some for later (again - scavenge-mode). I totally can understand it. If he had asked first, I'm sure there would have been no problem.
Thanks for helping me sort this out.
Becca