Brooke Mueller's mom claims there's an explanation for Brooke's bizarre behavior in the pawn shop yesterday -- telling TMZ, her daughter was probably selling off some stuff for a friend .... AS A FAVOR.
The good Samaritan excuse isn't passing muster with two of Brooke's friends, who tell TMZ she's fallen off the wagon and COMPLETELY disappeared.
But Brooke's mom Moira Fiore is adamant .... claiming she knows exactly where Brooke is -- although when we asked her about Brooke's possible drug use, she didn't know anything.
Also, we're told Brooke's cell phone is turned off.
TMZ got video of a very skittish Brooke at the L.A. pawn shop Thursday -- and things didn't look too good.