Where is the documentation on /tmp? I've read that /tmp will automatically have files deleted that are older than 7 days or upon restart, but I can't seem to find where this is documented. Can anyone point me to a man or web page from Apple?
There is a little documentation here on the background tasks that do things like clear up the /tmp folder, but it is marked as archived now and Apple don't seem to have released an updated version.

There should be a log at /var/log/daily.log for the daily script that (amongst other things) clears out the /tmp folder.