Hi Wayne,
I believe I answered this in email but thought I'd post here for others who may be wondering.
There is the "Date Last Opened" attribute. You can select a file in Finder and do "Get Info" on it to see it. It may or may not be reflective of your definition of "in use". For example, if you load a document into an app, it is "opened" at that point, but if you are viewing it for a while in that app, that is not recorded anywhere even though you consider it "in use".
You can try setting the delay long enough that chances are slim you are still working on the file. An alternative would be a warning system. After, say, 6 hours, color the file red as a warning. If you don't manually clear the label, then some time later, you check if the file is still red and then move it. Requires manual intervention but hopefully that would be uncommon.
So, those are a couple of ways of dealing with it. Let me know if you need more help.