More than likely fumes. Could be related to the CAT. Maybe it's leaking or is fubbared. When it's cold first thing in the morning put your head under the car and give the car a bit of gas to see if you can spot any leaks. Also have a listen for any blowing.
Check around the exhaust manifold and downpipe flange for any black soot which would mean the gasket is leaking.
Also worth trying a different petrol supplier. Sometimes there's just a batch of fuel which has a higher than usual sulphur content. Before the cat has fully reached its ideal operating temperarure this leads to a higher than normal % of hydrogen sulphide in the exhaust gases. When this happens it's harmless but smelly. Whilst it lasts you can eat whatever you want and if a passenger complains you can blame the car...
If it's only noticeable with the bonnet up and the smell isn't gone a few minutes after starting (i.e. once the engine has FULLY warmed up) then Ashy's suggestion of a possible leaky gasket would also be high on my list of possibilities.
Not really. None of my cars has ever had "eggy fart exhaust" and they all had perfectly good catalytic converters. OTOH I've encountered a few cars that did and in MOST cases there was nothing wrong with theirs either.
Did you manage to resolve your smelly exhaust problem?