2009 Mac Mini Wakes Up Immediately After Sleep. What Could be Wrong?

This computer would sleep fine before. Now it wakes up immediately after sleeping, either from a timer, or from the Sleep menu item.

• Even if all USB ports (except the wireless mouse) are empty


• Even with a non-wireless mouse


• Even if Bluetooth and WiFi are off


• Even if Time Machine is off


• Even after Apple Hardware Test has shown no problem


• Even if a B.S.O.D. has not happened during the session (See Black Screen of Semi-Death after Sleep/Fast User Switching )

I've checked the "PleaseSleep" logs and there's nothing there, I've disabled "PleaseSleep".

I've checked "syslog" from a terminal and the machine is woken up by USB7... but I don't have seven USB ports.