It's rare for someone genuinely not to notice a hit. The only time it would normally happen is if the BB skims off someone's pouch while they're on the move, or maybe touches the bottom of their boot. If they have plates in their carrier, those won't be felt, but they'll make a very certain "crack" sound, so it's just as easy to recognise and call a hit with plates as without.
You'll know when you're dealing with a cheater, because everyone around you will be saying the same thing. I had that yesterday - was with my sniper rifle and I was sure I hit an enemy in the chest a couple times, but he didn't call it. Someone else thought they hit him too but no call. I took another shot which I watched through the scope as the BB flew downrange and pinged off his bare elbow - he flinched, then went back to playing. If you have that experience (everyone does at some point), just tell a marshal and usually they'll sort it out. If they don't, then that's a bit of wise feedback for you about that site.