Every now and then I get a wiff of fumes coming in from the outside even when the recirculate air button is on. I have also noticed this turn off (there is a beep) - why does this happen?

 

The beep is to tell you that the recircis switching itself off after a certain period of time.
I would also check under the LH scuttle panel to make sure your pollen filter is seated correctly so fumes have to go through this and not by-pass it