They came by and you were unavailable. Personally, I don't like overbearing people and I feel that calling, texting, knocking and simultaneously announcing to the world that they tried finding you and you were unavailable is obnoxious. I would try just one of those methods (which they did) and if I didn't get a response, I'd assume the person is unavailable and move on. I would think that they are sleeping, went out for a drive and got delayed, needed to get gas or run an errand, are having sex, showering, etc., and would not want to keep insisting. If I needed to leave somewhere and start driving, there'd be more of an urge to hit the road, even more so for a 15 hour drive!
Honestly, it wouldn't even cross my mind that someone might be offended that I didn't try other methods of communication, needed to apologize, or bring up the fact I knocked and got no response. You shouldn't take it personally. They probably thought, "Well, they're busy/not home, we have a 15 hour drive, we saw our son and will see him again in a few weeks for Christmas, he won't be upset if we hit the road, he will understand if we leave without a hug, let's just go, we'll see him and his wife and kids soon enough" without thinking you'd be upset.
Do they usually hug and show all this lovey dovey affection when they leave anyway? Maybe it's just not a custom of theirs. Some people are very touchy and affectionate, others are more about having a lot of personal space at all times. Some cry at goodbyes and avoid them -- my former brother-in-law will leave in the middle of a party, sneaking out, without anyone noticing, because he hates goodbyes. What is their usual practice, when it comes to saying goodbye to their son? As I said, since they said goodbye to you and your kids, they probably felt they weren't going to offend or upset their son, nor did they figure that you'd be upset on his behalf. It doesn't seem to bother your husband though, let it go.