That's exactly where I disagree. You say realistically they shouldn't have expected Death Gun to know. That might be the case as long as they aren't taking him seriously. But if Kirito (let's forget Sinon for now since she's only playing a game) had taken this murderer seriously, yes, apparently there's a reason to tell somebody to take a murderer seriously, then he wouldn't have assumed Death Gun doesn't know but instead that he might know. That's how it goes. When you play a game, you think overly optimistically, it's just a game, after all. But when you are cornering a deathly opponent in RL (which this would equal with its real death), you think of everything that could go wrong. Not that I'd be any expert, but this seems like common sense.
Besides, they had been observing the bridge from that same spot for a good long while. It wasn't a particularly covered spot either (Sinon pretty much admitted as much by thinking Dyne should check his six o'clock), plus Kirito actually assaulted Sinon right there. They had talked all the time, no, make that talked and shouted. If Death Gun had been observing the area for a bit before walking in (he's a sniper, after all), he would have noticed those two easily. He wouldn't even have needed to cheat. Besides, Sinon already has some kind of special vision, maybe night vision, so who knows, DG might have even thermal imaging in that skull mask... He's leaking coolant every now and then!