Seat Altea Anti Theft Alarm Problem

Sounds frustrating! I had a similar issue with my old Renault Megane. Kept getting false alarms, especially after a power failure. Did you check the alarm sensor connections? Sometimes they get corroded or loose.

Alarm sensor connections, where exactly are those located? And what do you mean by 'optical locking'?

The alarm sensors are usually located in the doors, hood, and trunk. They're basically switches that tell the system if something's been opened. Optical locking means the alarm uses the car's indicators to flash when locking/unlocking. If the coding is wrong, it can cause the system to malfunction. If you're not comfortable checking the sensors, it might be best to get it checked out at a workshop to avoid immobilizer issues.

Took it to the shop. The error code pointed to the alarm being coded for optical locking. They recoded it, and it's sorted. Cost me 45€. Thanks for the help!