It’s water in the camera. Common problem in the UK: telltale point is pattern changing as the camera cools.
You need to dry the camera out. Putting it in a warm, dry place for some days might suffice, without having to take it apart.
My preventative solution to this is to keep a dew heater round the nose of the camera and switch it on at least 15 minutes prior to imaging. Sometimes I’ve kept the heater on for days to dry the camera fully.