Stand-up heating runs sometimes also after/during the journey !?

Hello, with our XC60 BJ 2009, the heater is now running at some point after a short or longer drive and not only when it was programmed for the morning. Yesterday nothing was programmed and we drove about 50 km. When we got out, it smelled very much like exhaust gas and you heard the heater run clearly outside and in the vehicle an “electronic” fiepen. The Fiepen then stopped and also the heater turned off. The same showed up on the Back home (again about 50 km). About 14 days ago he showed this behavior for the first time, but unfortunately our workshop could not understand this behavior, because there did not occur. Does anyone know this behavior? Greetings Merlin