What happens if I spawn inside someone else in DayZ

If you have a high desync with that computer, it suggests a problem with networking/firewall on that machine. Do you have antivirus scanning packets going in/out? Maybe try adding the arma2 executable to an exception list (do not scan things that program sends out / receives).

Otherwise I have noticed I keep items sometimes if I use them and immediately logout. Similarly, I have lost items that I have found by logging out shortly after finding them. Make sure after looting / using items you wait five minutes or so before logging out. It may also be a problem with the dayz server you are playing on (or dayz version, or arma2 beta patch version).

Have you tried using Dayz Commander and updating arma2/dayz to the most recent versions? It is pretty painless to do so through Dayz Commander.

If none of this fixes your problem, I would seriously consider asking this question with more detailed technical specifications on the official dayz support forums. You may have found a legitimate bug in the game.

As many players specifically watch for bases/hidden caches etc, the only real technique to protect your stash is to work in a small group of individuals to have continuous activity on the server.

Of course, you'd need to trust your little clan...which is perhaps a separate issue :-)