probably nothing all boilers emit a small amount of unburnt gas each time the burner comes on this can be smelt a fair distance away and is often mistaken for a gas leak especially on calm windless days if you are still apprehensive and the smell is permanent and not intermittent get the system checked
brian
January 2011
I can answer this question...
Vokera Unica HE boiler shows a smell of gas coming out of the flue. ?