Did optimus actually ever find out? At least imo I believe optimus never found out iron hide died at least not during the film. At no point do we see any of the other autobots mention it to him which is why there was no mourning but I do believe optimus did feel that something was wrong. You can see it in his eyes when he returns to nest like someone was missing. This is why there was no mourning or at least no emotional scene with optimus over ironhides death. At least imo.