Rumer Willis -- The Demi Moore Rehab Mystery

Rumer Willis knows if her mother Demi Moore has entered a rehab center for substance abuse ... and Saturday night she MAY have revealed the truth on camera.

Rumer was arriving to Comme Ca restaurant in West Hollywood ... when a paparazzo asked her, "Is Demi in rehab?"

It sounds like Rumer says, "No" ... but her purse was over her face, so it's hard to tell if Rumer said the word ... or if it came from another woman in the area.