C240 Alarm goes off when locking the car
 

When I lock the doors the anti theft alarm goes off. This started happening just yesterday (I have 39,000 miles on it). Therefore, I cannot lock the doors, unless I want to hear that alarm all day.

Has this happened to anyone else?

could be a door jam switch or probably a bad alarm siren. alarm siren is behind your right front tire behind wheel well liner. or some electrcal problem . but the sirens are known to go bad.

Thank you for your answer, chassis221!

Update: today is the first day that the alarm did not act up. Let's see how long it lasts.