Well mirage of a sort, but a different kind, so to speak.
Outdoors, nothing shows with flags, your only option is to wait out thermals coming off ground . Indoors entirely different issue.
As a for instance, awhile back I used to fly RC hand launch gliders, very sophisticated ships, mostly carbon, 60" spans that weighed 10oz. Including miniature gyros. Point to this is that quickly you learn how thermals coming off the ground form, most of them within the first 3-4 feet off the ground and they can be surprisingly forceful and you will never see them through a scope. I learned a pantload about rising air when you are gliding 3-4 feet over the earth....hit one, and if you learn to stay in it , bingo, you're 30-40 feet in the air.
I remember the great series of articles published in Small Caliber News by Frank Tirrell (sp), all centered about precision rimfire, including his construction of a great rail Setup.
Anyway, Frank , in AZ built a 50 yd tunnel above ground but early results were simply horrid. Essentially the same issue we're talking about here with air within the tunnel heating.....creating several temp layers within an above ground tube 3 feet or so in diameter. He ended up resolving the issue with a complete air evacuation system which kept the tunnel at one, more or les, temperature .
Cannot imagine the cost of, say a 30-40 bench facility to install and effective evacuation system of that magnitude.
I'm guessing that's why the old Texas warehouse PPC tests were pretty neat. Space was protected enough to be great, but big enough to avoid these issues, also why, now guys try and put them underground if possible.