The Risk Profession, a 1961 novelette by Donald Westlake, available on the Internet Archive as a scan or in the text transcript.
The narrator is an insurance worker investigating the death of a miner.
And then it hit me. The whole thing hit me, and I just sat there and stared. They did not carry extras, Karpin and McCann, they did not carry one item of equipment more than they needed.
I sat there and looked at the place where the dead body was hidden, and I said, “Well, I’ll be a son of a gun!”
He turned and looked at me, and then he followed the direction of my gaze, and he saw
what I was staring at, and he made a jump across the room at the revolver lying on the cot. [...]
Two atmosphere suits. The dead man was supposed to be in one of those, floating out in space somewhere. He was in the suit, right enough, I was sure of that, but he wasn't floating anywhere.
A space suit is a perfect place to hide a body, for as long as it has to be hid. The silvered faceplate keeps you from seeing inside, and the suit is, naturally, a sealed atmosphere. A body can rot away to ashes inside a space suit, and you’ll never notice a thing on the outside. [...]
I studied the two atmosphere suits, at the same time managing to keep a wary eye on Jafe McCann, standing rigid and silent across the room. Which one of those suits contained the body of Ab Karpin?
The one with the new patch on the chest, of course. As I’d guessed, McCann had shot him, and that’s why he had the problem of disposing of the body in the first place. [...]
“Think I should open it up and check? It’s been almost a month, you know. I imagine he’s pretty ripe by now.”
I reached down to the neck-fastenings on the fishbowl, and McCann finally moved. His arms jerked up, and he cried, “Don’t! He’s in there, he’s in there! For God’s sake, don’t open it up!”
Found by searching this site for
[story-identification] asteroid murder which brought up Story about an insurance adjuster from outer space.