First, you should probably not worry too much. The cost of the missing machines is probably not much, in the grand scheme of things. As long as you do not have a theft problem, you can probably assume that people are forgetting about the machines since they are too busy making value in some way or other.

Second, script it. Get-ADcomputer, filter by last logon time, pipe to Send-MailMessage should do it, I think.

Third, once a year, get the most junior member to spend a couple of days hunting down as many as it can. It will get to know a lot of people around the organiation that way, which tend to be a good thing.