Check out inotify tools, or if you're a programmer, you could roll your own that fits. It's not that hard to do; the most difficult part is keeping track of all the subdirectories, and dealing with directory additions/deletions/renames.
I have inotify-based programs running on a few of my servers. For example, on the one that stores my scanned private documents (bills, receipts et al), I have a program that watches for new files in a directory tree. When a new document is created, it is immediately PGP-encrypted (unless the new document IS pgp-encrypted, of course). Another, similar program, sends any changes in a particular tree to another server, far away.
I could see modifying one of those to simply write to an audit file that could then be reviewed as need be. The most difficult part I see in that, is making sure the audit file doesn't grow too large.
I will review one of these to see if it's public-ready. If it is, I will find a place to share it.