Using a Bushnell holosight on a flattop m4gery, I've been getting a consistent spot of fogging/condensation on the inside of my holosight. The fogging appears as a 5mm wide streak on the inside of the front window, and almost always at the same point in time during a range session. As I let the receiver cool, the fog fades away.

This leads me to believe that the heat of the upper receiver, when adequate, heats the holosight enough to cause moisture inside(!) the holosight to condense on the interior of the glass, due to the temperature difference with the outside air. Humidity conditions have no effect on this.

I also see "spots" of some type on the interior of the glass - and given that these sights are supposed to be sealed, I'm a bit curious as to how anything could get in there. I'm guessing the seal has been ruptured somehow.
Which is odd, since I treat my stuff with the utmost care.

Anyone else ever have this happen?

Just soliciting opinions before I get it serviced. Any advice would be great. Thanks.

I had a condensation problem inside mine in the rain (nothing quite like shooting in the rain at the range when you're not even being paid for it!).  I sent it back to EOTech and they serviced or replaced it.  It was obviously not sealed and nitrogen purged properly.  I checked it in a hot, steamy shower when I got it back - good to go (although the wife was not amused).

Call EOTech, they will try to solve your Bushnell problem for you.