Squirt some CLP in the striker channel and dryfire the snot out of it.
Bet it goes boom shortly after.
If it does, completely strip the gun, clean it thoroughly and enjoy your "slightly used" [sic] M&P.
(canebrake would go back and kick the seller's azz!)
Get her dirty, then clean her so she starts to respect you. When her trust is complete, she will serve you well for a lifetime!
"...if doves shot back, there wouldn't be a need for a bag limit." - orangello