Oil fill cap/temperature question

[APXD 30]

Over the weekend, I checked fluids & tire pressures on my cars.    The GTI was half a quart low (not unusual for this car).   When I removed the filler cap, I found what I can only describe as a 1/4-1/2 inch dried foam on the bottom of the cap.   It was a sand colored substance.    The car hadn't been driven long this weekend but moved around once or twice.

The dealer said, over the phone, that this wasn't a big deal and that it was the sign of a hot engine cooling rapidly with the low temperatures.  They didn't see a reason for bringing it in.   The Audi was not this way but had been driven about an hour before checking it.

Obviously, I will keep an eye on it and take a picture of it next time.   Maybe I'm hypersensitive b/c my A4 1.8T had the sludge problem on the first engine.

Any thoughts from someone who might know more about these things would be appreciated.