Tyre Swapper?!

My girlfriends cheapo bike that was literally given to her for free, is kept outside, and although it is D-locked the front wheel is left exposed.
Last night though, someone took the front wheel off, swapped the tyre and put the wheel back on the bike.
I noticed this morning it had a different tyre.
It now has a cheap slick tyre instead of a offroad one, but who would do such a thing ?
Weird ! Things that happen in London honestly.

As someone with experience in these kind of magical events, let me break down to you what really happened..

1. "Let's just go for a pint"
   
2. 8 pints later, "SHOTS!!!!"
   
3. "&^$ shots later, at home, now tired of midget-fondling you decide some bike maintenance would be a good idea.
   
4. Tyres swapped, you puzzle over what to do with the knobbie.
   
5. Your neighbor is now puzzling over where his FIAT Punto front-left has gone but appreciates his new mtb tyre.
   
   

...