How about creating a list 'You know you've stayed too late when.....'
You can't have made these up - they must have happened to you or someone you know well (no 'the friend of my brother's girlfriends cousin twice removed' sort of thing).
To start - all these three happened within the last month (the first today).
The porter says 'what the hell are you still doing here' and he's not joking
You leave the office and find the main building locked up and dark
You leave the office and find nobody else on site
You go to the main building and walk into a locked door
anyone got any others?