Lamp warning on , couldn't see a problem , scanned with VCDS , left hand rear brake lamp at fault , looked ok transposed bulbs , light went out cleared code happy days , then had warning intermittently. Checked out wiring and bulb carriers etc , fault cleared then boom on permanent. Brake light still the fault but working , notice rear upper small side light out. Changed that light still on , then noticed front l/h dipped out !! Replaced that as well light out on dash. Scanned again and said fault was l/h brake lamp. Cleared codes and so far so good !!

It has been reported on here quite a few times where a bulb warning is on despite the bulb still working. It was determined that a bulb beginning to fail, or failing intermitently could be enough to trigger the dash warning.

 

The two W5W bulbs that make up the 'C' shape on the tail lights are not monitored by the blown-bulb warning system, hence you see quite a few Octavia's with them not working.

 

Also headlight bulbs tend to fail at the same time so you can expect the other headlight bulb to pop soon!

I would replace both rear brake lamp bulbs at the same time as they are now that expensive.  Volvo's used to be plagued with this problem if you replaced only one bulb as the system was so sensitive that it could tell that one bulb had a higher resistance than the other and so it thought that one had blown.