Something and nothing i think, or is it?

My pc, like others id imagine, goes into standy mode. The fan stops and its like it is switched off, and just pressing any key fires the fan back up and in seconds the screen is back on.

Lately though, the fan stops as it goes into standby mode, but immediatley its as if someone has pressed a key and it fires back up!

Any ideas? No we havent got a ghost lol!