Legal Question in DUI Law in California

Officers can be anywhere that is not illegal by code. Here there is no code saying an officer can or cannot wait outside an establishment that sells alcohol.

You may have other legal arguments.

The law regarding search & seizure is complex, and it's all based on the Fourth Amendment to the United States Constitution.

In order to stop a motorist, the police officer must have reasonable cause to believe the person committed some sort of offense, even if it's something rinky-dink like a burned out taillight.

There is nothing that prohibits a police officer from waiting outside a bar and following motorists who leave, hoping they will do something that would justify pulling them over. Of course, if someone stumbled out of a bar and appeared drunk, then got into a car, the officer would be justified pulling that person over, too.