Well if anyone is naive enough to think that because it's a main deler their car is 100% safe then thats their own opinion. Who do you think works in these places, average everyday people, and in that mix you will undoubtedley get some dodgy characters who will play a hand in stuff like this.
I don’t really think it’s really that hard to speculate that it’s someone that knows him or the car, 3 possible scenarios
1)Person heard he was getting his car MOT’d and followed him there.
2)Someone that worked there tipped someone else off.
3)Pure and utter coincidence, which I don’t believe.

(Based on the fact they wanted to steal that exact car with that spec. how would they know which garage and when to go and get it) Hence why I believe it’s 1 or 2. Anyway makes no odds to me but I hope he gets it back in a reasonable condition, but I doubt it. )