Egg smell

Hi guys

Looking for some advice please.

We have a 2019 118i (front drive) with 16k on the clock.

All servicing is up to date.

We’re getting a very slight egg like smell through the air vents when the engine is hot. It happens after we’ve slowed down or stopped, eg at lights and are then pulling off. It clears quickly and is fine when driving at normal speed.

We had a transmission oil service recently and thought it may be that some old oil had spilt but that’s been checked and all is fine. We’ve also had a visual inspection of the exhaust and were told it looks like new but they couldn’t see the cat as it’s covered by the manifold. But surely a cat shouldn’t fail at 6 years / 16000 miles?

Has anyone experienced anything similar? The smell is minor and the dealer can’t pick it up, but it’s definitely there, both my wife and I can smell it (and it’s not us!!). Any thoughts would be appreciated.

Thanks all.