Keanu Reeves -- 2nd Home Intruder in 2 DAYS ... This One's Naked

It's borderline incredible ... ANOTHER intruder made her way inside Keanu Reeves' house ... and this one got wet and wild.

It was the SECOND time in 2 days ... a woman waltzed into Keanu's home through an unlocked door. Keanu was gone ... a cleaning crew inside the house left the front gate open and never flagged the intruder.

Law enforcement sources tell us ... she got naked and jumped in his shower, then took a skinny dip in the pool. Only then did the crack cleaning crew get suspicious and call Reeves, who called 911.

He certainly knew the drill. The day before another woman broke into his house when he was sleeping. He found her in his library and talked to her briefly before calling 911.

Both incidents went down last week, and both women were taken away for mental evaluation.