The idle solenoid vibrated out of its place and went haywire due to bad ground connection. Fixed it but now the battery is showing 13.2-13.8 volts when the car has been turned off for a couple of hours, and it is hissing slightly.

I checked the voltage when running and it fluctuates between 13.5-14.7 so the regulator is not broken.

Is it OK if the battery is above 13v at times?

Sorry, I have no idea but hissing suggests its boiling which can happen when charging if its knackered.