Serena Williams locked in Panic Room Over Drug Test Mix-Up

Serena Williams  went into total lockdown last week -- fleeing to a panic room inside her L.A. mansion after security cameras picked up a suspected intruder ... but it was all a case of mistaken identity.

Law enforcement sources said, Serena's assistant called 911 around 6AM last Wednesday, reporting a possible burglar or prowler. It was so serious, we're told Serena retreated to her panic room for safe keeping in case things got dangerous.

But we're told, it was a false alarm -- the person on Serena's security camera was a random drug tester for an unspecified tennis association, stopping by Serena's pad unannounced for some pee.

Police rushed to the scene after the 911 call, but quickly figured out the mistake. It's unclear if Serena eventually submitted to a pee test. Attempts to reach her reps were unsuccessful.