I think 95% of 'Tuner Shop' break-ins are committed by people who have been to the shop, most likely in the last 2 weeks. Possibly 'hanging around'.

Spec-R, XAT, DC, Keiths shop (don't remember the name)

The only 'Tuner Shop' that I can think of that was burglerized by a random person was Racing Zone back on Nebraska...somebody did a smash and grab by ramming a pickup into the building.

I bet it's a customer, or somebody that been hanging around lately. 50/50 chance they'll even stop back by during business hours to throw off suspicion. But hey...what do I know?