I can't remember where it was from, but basically it is after the war and the mercs all get together. Pyro has dies and Medic steals the body to analyse it. Turns out that pyro was a robot the whole time and Engie is devastated I would appreciate it if anyone knows this one, I have been looking for it forever...
https://m.fanfiction.net/s/9553705/1/ I guess this is the fic you are looking for :)
That's it! Thanks :D