Another thing, which has sort of been pointed out before... parents can tell if you're looking because you understand and want to help, or if you're looking because you think the kid is a brat. You could be there with a completely blank expression on your face

and the parents can still tell if you understand or not.
We have that happen w/ Justin alot-- I suspect all of the parents do. If you just see him, and don't watch him for a few minutes, you can't tell anything is going on, depending on the situation he blends in perfectly well for a good chunk of time, until he decides (usually) to start flapping. So we will be in the store or out somewhere... and you can tell the moment that a stranger figures out that "something's wrong"... it's kinda funny to watch, if you have a sick sense of humor.

The facial expression changes, but they're really
trying not to let you notice.
Which, back to the OP, means that it's very likely that the parents did see you, and did notice that you noticed, and could tell that you weren't being judgemental. Right there is a big thing. It also means that if they needed assistance, you would have been the person they turned to. We can just tell, the people that understand and the people that don't, without them saying anything.
One thing that's come in really handy for us, is having a big-mouthed older brother. He'll tell anybody anything, and if someone is giving us the "evil eye" he's quick to point it out, loudly. Now, that's funny stuff. Nothing like a 9-year-old telling an adult that his brother "has autistic" and to quit looking mean.