I have caught my husband out TOOO many times with this!!! We are driving along, chatting happily, when this smell, best left to the imagination than me describing it - believe me!- invades the car. I try to be subtle and open a window and he instantly goes, " Paddy!! What on Earth did you feed HIM last night!!!???"
All I have to say is, "I knew I forgot something!" for him to realise the dog is not even in the car!