I wouldn't mind it if fireteam weapons could be trophies and it'd be an easy kind of trophy to add. The implication that past teams have tried and failed is a good way to intimidate the enemy. It also works in-universe since we know a few Predators are operating in the same jungle meaning it should be reasonable one of them picks up a modern pistol as a trophy. No more weirder than a Predator taking a flintlock or revolver or even a knife. It's like the Human skull, just an obvious and easy thing to add.