I think it’s because people can never find their keys.

Jesus I thought that was a reindeer stuffed animal they tucked into the sleeping bag to make it look like someone was in there.

I was way off base.

I presume because of the keys people will think he’s a drunk that passed out instead of a dead guy. Hence they won’t want to be involved.

Maybe other people will take the keys to overtake the dead person apartment, and hide the body so they could keep living there, letting the OP murderers get away with it.

Am I overthinking the joke?

I took it as the saying if you lose your keys or other objects that “they are always in the last place you look”