Occasional Third Brake Light failure message

Very occasionally on startup I'll see a message to check third brake light.

The first time it occurred was during a frosty evening in March, then it occurred this morning after it was parked up in yesterday's heavy rain.

Stopped at a busy junction I quickly turned the car off and on again (the IT diagnosis approach) and it went away.

Is this a sign of water ingress causing a bad connection, faulty seal etc.?

Thanks