If a decoy is within like 8 meters or something it hides your glint. I think that other classes using sniper rifles don't have a glint either. There's nothing new I'm aware of in S3 that changed anything regarding this. I think they claimed to have done something to stop decoys from just phasing out of existence when placed, but that was a lie because it still happens.
And yes, it's completely OP and allows you to set up shop in good areas without just getting blasted for pressing RMB. Though there are still bullet trails that will give your location away, and a console player just spamming ADS will eventually just lock on to you through god and they can blast you across the map with any weapon.