I have  2004 Legacy 2.0 auto which is getting long on the tooth, but has generally served me well.

However, these past few nights I have found that if I lock up the car with the IR button on the key, some hours later the car starts emitting a siren noise for about 15 seconds off, then fifteen seconds on, for up to five minutes.
No indicators flashing - just a siren.
If I try and unlock the car, this works, but the alarm still sounds. I can even start the car and drive it around - with the siren still going, so as well as no flashing lights, this siren noise is not associated with the immobiliser function, either.

On the first night it happened, I hastily got a torch, flipped the bonnet and removed the +ve connection from the battery - I could see no other way of shutting the car up!
This resulted in the car running very badly the next day and stalling for a few hours - I believe this is because the engine management has to 'relearn' the engine's habits.

Does anyone know what this issue could be?
At present, the only option I have (other than to disconnect the battery) is to leave the car entirely unlocked overnight. Not ideal!

If you lock the car with the key (not via remote) so the car is locked but not alarmed, does the same night-time alarms still sound?

Sometimes one or more door switches, maybe one of the bonnet plungers might be sticky causing the system to think the door/bonnet is open? Worth checking. The only other possibility i can think of is the alarm's 9v battery needs replacing. Found under the scuttle near the nearside wiper mounting position.