I suppose it's the more obvious things that draw their attention, like rashes, coughs, etc. And, the reality is, as common as those types of symptoms are, they are also things we've told them to watch for.
It's bad enough that we, as parents, have to find that balance between vigilance without paranoia, I wish they didn't need to find that balance too. I know this weighed on S over the past year - between his rash last year, chest pain, cardiac questions, back to rheumi related, colds/bronchitis and his shoulder issues in between, a number of times S mentioned that he was feeling down and upset that he always had health worries weighing on him.
Hopefully, this is just part of the transition in becoming an adult. In years past, these worries were just ours and we eventually learned what could wait but now, while they're more aware of the symptoms, they just haven't enough experience yet in balancing what can wait or what needs to be seen right away.