I have noticed this smell before and with all four of my cars. In pretty much all cases, the smell is encountered when reaching the top of a long hill on the interstate. I do think the smell does not come from my cars but rather from other cars' exhaust.

That long climb heats the cats and burns off the accumulated gunk in there. The reason I say it does not come from my cars is that it even happens on two of my cars which do not have catalytic converters. It is especially noticeable in my Cobra which is a convertible where you are all out there in the air (and exhaust).