The compute module is under the back seat in the Model 3 I believe. That's all of the major components, might be tricky but someone who doesn't care about damaging the seat may well be able to get in unnoticed while riding in a Tesla taxi.
It's in the dash. You have to remove the glovebox and several pieces of the dash. For some reason I think the taxi driver would notice you disassembling his car.
It is behind the glovebox
Is access to it lockable? Would be useful for a taxi usecase
In the model 3, the only way to access the glovebox is to press the button on the console. I'm sure this would be disabled when in self-driving taxi mode.