Spice works will keep a timeline on changes to your network inventory. Anything installed or uninstalled, added or removed from your workstations, DNS changes, etc. It's quite a nice little tool. Of course, it's not going to come out & tell you something was stolen, but it will let you know if anything has been uninstalled or removed.
In Spiceworks, you need to run a report of all machines with total memory amount changed in last week, month, etc.