I'm going to be boring here...
At my place of work if somebody uses a computer that is logged into another person's account then both they and the logged in user would be subject to a security investigation, the initial phase of which will involve their account being locked and removing their access to the IT system.
Not only that, but unauthorised access to a computer and unauthorised modification could be seen offences under the Computer Misuse Act, which could have negative implications for your IT career.
Maybe instead of pranking somebody it would be a better idea to just press Windows-L.
I said I was going to be boring.