Good that you replaced the main vehicle battery, this eliminates it as a suspect
The fact that you do not have a sunroof is good because it means you never have the risk of water damage to the CEM (Central Electronics Module, one of the main 'brains' of the vehicle)
Note that disconnecting the vehicle battery, waiting 15 minutes and then reconnecting is similar to rebooting a PC: it reboots the vehicle. This will often clear random faults.
The volvo factory alarm system sounds the vehicle horn AND has a siren with it's own internal rechargable battery
The fact that you do not recall hearing the siren means that the siren may be so faulty that it no longer sounds reliably.
The siren battery fails after a few years - somewhere around 10 years old - and one of the symptoms is the siren sounding all by itself.
The siren is connected via a digital LIN databus to the Upper Electronic Module (UEM) in the roof near the rear view mirror, and the UEM connects to the vehicle CAN bus.
A confused siren could possibly confuse the vehicle. Shouldn't, but could.
The siren is powered by fuse F8, located on the CEM.
The CEM is located on the left hand side lower area of the dash, above the footwell. To access it, remove the lower dash cover (which includes the courtesy lamp), and then locate the CEM
Videos here:
http://xemodex.com/blog/volvo-centra...70-v70-s70-s80
Locate F8 and remove it, that disconnects power from the siren.
Should the siren battery have some charge left, it may sound for a while until the battery is drained.
Then consider repair or replacement of the siren....
OR
Maybe you have a CEM fault caused by deteriorating solder joints.
If removing F8 does not stop the problem with vehicle remote unlocking reliability then consider sending the CEM in for check and repair to the London guy that specialises in them:
http://volvodiagnostic.com/shop/inde...duct&id_lang=1