Take it to a dealer and have a software reset done. Then wait to see if it ever happens again.


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On the older P2 era cars when the alarm goes off like this it typically means the battery inside the alarm module is shot. Unfortunately, that typically means replacing the module as that battery typically leaks and the circuit board inside the alarm module gets wrecked by the leaking battery acid.

I don't know if that occurs on P3 cars so YMMV.

 

The cause of the alarm activation can be read using VIDA. Recent alarm activations are recorded by the CEM, including cause, status information, time run on.
It would be easiest to diagnose it using VIDA as this will tell you exactly what is happening.

There isn't an easily determined root cause in these cases. Common, as in does happen on occasion, causes can be:

1) Failure of any of the 5 locks. Failure of the locking motor or dead locking motor can either cause improper locking (door not actually locked, will set a corresponding DTC) or unintentional unlocking. Improper locking should on paper not set the alarm.
2) Failure of any of the microswitches in the 5 locks. This will cause the car to think a door was opened, even though it is still closed and securely locked.
To repair is either to disassembled the lock and replace the component in question or install a new lock.

3) Failure of the bonnet lock/catch microswitch. Alternatively failure of the bonnet lock/catch microswitch wiring. Intermittent contact can cause unintented alarm activation. Can cause bonnet open warning to appear in cluster despite bonnet not open.

4) Siren battery failure.
5) Main vehicle battery low / failure.

6) Depending on equipment level: Inclination or interior movement sensor detecting something -> alarm goes off

Locking the car with the mechanical key will not set the alarm. Can still be unlocked using the remote.

 

Thank you all, and particularly blockparie.

Lots of great info here. I'm going to try to have it scanned with vida and see if it can narrow it down.

Hopefully it won't be the ruined alarm module due to battery leakage.

If this is the case, can I just pull the battery and run without it? I feel like car alarms.dont really help anything anyways. They are so common these days that everyone just ignores them and gets pissed over the bother.

Much obliged,
Matt

 

The siren battery can not be removed entirely. Everything is more complicated than it sounds:

Design wise the siren battery is contained within the siren assembly. Volvo does not sell individual parts for the siren, only complete replacements. Before everyone gets angry about this, it does not make sense for a manufacturer to provide everything in pieces. Nor would it be better for the consumer as the higher cost of making everything user serviceable would be passed on.
The siren design is of the "no user serviceable parts inside" type. Which means that the casing comes apart with some force as top and bottom are only clipped into each other. At that point it is easy to visually inspect what damage has occured to the siren. If the battery has leaked the acid eats away at the circuit board and in most cases damages it beyond repair. If things are still okay just buy whatever battery you can find with the same specs. There are two wires connecting circuit board and battery making this a simple solder job. Or crimp job. Or bodge job if you use electrical tape. Which you shouldn't.

If the siren has to be replaced only use a numbers matching replacement. Several variations exist and there are two general versions: With and without inclination sensor.

 

The battery is part of the siren module so the module would need to be unplugged. That would most likely result in a warning light so on the P2 cars it is possible to get a software download from the dealer to deactivate the alarm and remove the warning light. First step is a VIDA scan and then go from there.