Horn beeps twice when unlocking

[vkostakos]

Hi folks.

I recently bought a 1999 Boxster without an alarm or keyless entry but with central locking. I have observed a strange behaviour when I unlock my car using the key:
-If I turn the key left, both doors open fine.
-If I turn the key right then left, then both doors open fine but the car beeps twice.

I wonder if the car is confused about not having an alarm, or could something else be causing this? The manual says that two beeps may be due to a fault when locking the car, not unlocking.

By the way, locking my car produces no beeps.