Mini!Prowl being picked up by you during a mission to get him out of danger.
You apologize for the sudden appearance and such, but his processor is on…other things.
Your chest in particular.
Nothing scandalous, but rather, simple curiosity. Cybertronians don't have soft skin like humans do, nor do they have certain anatomical features.
Your heart, he could feel it…beating. It fascinates him how different humans and Crybertronians are. So don't be surprised if he asks to listen to it once more...
Or maybe a couple more times.
At a certain point, the others will see him resting his helm against your chest, always asking what's so interesting about it.
You explain it's just Prowl being curious about human hearts, and while that isn't untrue, he's come to like the feel…of your chest…but he's a bit too shy to say that, so he'll let you speak on his behalf.