All the sniper has to do is stand under a big tree -- anything to block the infrared and other emissions. A flash suppressor would help also. A parking garage roof or something would be even better.
I also suspect that an urban environment generates so many false positives that muzzle flash locating would be less useful. No so in an open desert environment, for example.
I thought that it could still be seen if he was outdoors in a stand of trees - didn't realize all the ways the flash could be suppressed or hidden. I was hoping this would nail them.