Not sure if this helps, but to get them all back. I had one admin suggest that at the end of the year (or when you want them back), disable all Chromebooks in management, with a message to have them bring them in to get them activated to be used. They will either bring them in, or toss them out since they never intended to give them back anyways. This is an extreme measure, but it will help locate all "lost" Chromebooks.
I think it was GeekyLibrarian who said she was doing that with hotspots at her library, and it did lead to a return in a day or two.
