I can not think of a good reason why anyone would have been in the electronics that compartment while replacing a battery. I would chalk this one up to good fortune... and investigate other BMW shops in the area.

Seriously though, I would go right back and talk to the shop manager about what you found. I would try to read the manager's reaction and see how seriously he takes it. If the manager does not seem to accept it as a plausable possibility that one of his technicians did a little extra work, then I would go elsewhere.

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