With my game group of a few years ago, I saw an unbelievable scenario happen twice. The PCs didn’t check for a hoard after killing a dragon. This was the same Players, but two different campaigns.
First scenario:
The PCs were in a swamp, looking for a structure to delve into. They found the structure. As they neared it, a dragon leaped up from behind it. The fight ended with the dragon dead.
The dragon’s nest and hoard were behind the structure, where the dragon leaped up from. It was not hidden. But the party didn’t bother investigating around the structure. (Just walking around it would have found the treasure.) They just went up to the front, opened the doors, and went in. They explored, came back out, and went home.