If amyone remembers that in Season 2, the Episode 'Changes', Heatwave had said "it feels like Boulder ran me over.", well, here is one of those times it happened (because I really think this would happen).
So, obviously eating that 'magic meat' wasn't too great of an idea. So while everyone was chasing the two monsters, a little commotion had happened at one point. Heatwave did find the one, at the same time as Boulder, and everyone can guess what happened there.
Did anyone notice the markings all along Heatwave? No. Did anyone ask how the hunting went? No. Did Boulder, Heatwave and Kade agree to never mention it to anyone? Yes, they absolutely made sure of that.
In the long run, although it seemed like barely anything was done when it happened, Heatwave was in pain for an entire week. He struggled trying to keep himself in a proper posture as the back parts of his frame were more than likely bruised, not to mention lifting some things were a hassle because of it.
No one noticed this but Boulder and Kade, but they swore to never bring it up since someone would ask about it.