Wait a second. Your thread title is about mirage...but then you use the word "fog"....
It is very common that a string of shots will heat the barrel enough to where the heat wafting off the barrel will create nasty mirage through your scope view (as it's right in line with it). This can happen with any type of handguard or tube. I've seen people use tin foil to avoid this at the range..they lay a whole sheet across their handguards and barrel. slightly folded like an upside down V or basically like a roof profile.
Fog would mean that the scope is damaged, ie, the nitrogen leaked out and conditions created some type of fog or condensation inside/outside the scope. I find this hard to believe on a scope like that.
BTW, the dude telling you about the 1/2" group is delusional. Someone that out of touch with reality, or that dishonest isn't someone I'd want to shoot next to.