So I've seen around a few times that Danny's heart stops and freaks out people with superhearing, particularly Superman and his family. Que the freaking out because this child's heart has stopped and they haven't even seemed to notice.

But what if Danny keeping his heart beating takes effort? He has to focus on it at least a little all the time to seem more human. But if he's relaxed because he's, say, gaming over at Tucker's, he stops paying attention to if his heart is beating or not, and it stops. Sort of like how if you're walking around in a place you don't know in the middle of the night, you're tense and worried, but once you make it to a little area or a place you know better you stop being so nervous.

All that to say, having a beating heart is a stress response for halfas. And, like any other stress response, having it be constantly active can have long term negative effects. This was discovered by Frostbite shortly before Danny told his parents about being a halfa, and in fact influenced that decision. It turned out well, and Danny's heart began to stop more and more frequently while at home, to the point that it rarely ever beats at home now.

So que Phantom joining the League through some shenanigan or another, and together they're working to dismantle the Anti-Ecto Acts. The League knows he's powerful, but he's paranoid enough that he hasn't told them about the whole "half dead" thing. He's wandering around the Watchtower, chatting with Superman as they walk past a window that looks out into space. Danny's chatting about his weekend plans with Sam and Tuck, Superman's chatting about his upcoming visit to Ma and Pa's farm, and suddenly the kid's heart just stops.

And Superman freaks out. Because there is a child right next to him having a heart attack and seemingly not knowing.

He rushes Phantom to the medbay, only for his heart to be beating again? And the kid looks really freaked out and confused now. So he apologizes, explains what he heard, or rather didn't hear, and asks if it's an arrhythmia or something.