How to detect any changes to a database (DDL and DML)
One idea would be to make a snapshot every day and monitor the snapshot file size on the disk using a file monitor. The snapshot is increasing its size only when data is added there, so it would be a valid idea if you would find a tool to monitor the real size (reported size).
Now.. I didn't use this, so can't give you technical insights :-).
Another idea would be to verify the transaction log of each db (if you're using full recovery mode on them, of course) with some function I've seen on the forums (db_fnlog.. or something) that reads operations from the log, and see if you have any deletes/inserts/updates.
Those are no easy things to do.. but I hope you'll find them useful.
PS: found the article with the log read function (it's fndblog, by the way :-): Read the transaction log by Jens K. Suessmeyer.