Car alarm going off after getting new (used) engine and new alternator belt this week

[Maz2010]

Halp! My car alarm is going off randomly. Based on my research, there are a myriad of things that could be causing it. I know nothing about cars and really feel in over my head. My car was running great until last month when...
-alternator belt broke, causing water pump to break and engine to overheat.
-Took the car to the (non-mazda) mechanic, they replaced the alternator belt, and put in a new (used) engine and water pump.
-Picked up the car this week, but overnight the car alarm started going off randomly.
-Took it back to the mechanic, who put the car on the 'sensor', and they say nothing is wrong.
How do I proceed? Thanks

[S.F.W.]

Likely not directly related to recent repairs. Most likely hood latch is rusty or broken.

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[Flunk]

This happened to me with my old 2010, stopped after I cleaned the latch for the tailgate.

I'm with him ^^ clean the latches.