I'm doing a series on alternate movement forms available to supers. Some of these things you will have figured out already, or really should have. Others might be new.
Flight is the dream of man. Gods, wizards, and ancient heroes were all able to muster this power in the stories. A lot of supers have it, via a lot of different options: control over fundamental forces like gravity, some sort of force field or barrier, actual wings, and so on. Almost all supers who can fly also have the required secondary powers necessary to use it effectively: resistance to oxygen deprivation, extreme cold, and so on.
The red line for humans is about 26,000 feet. Past that, an unmodified human will basically suffocate to death because they can't take in enough air to oxygenate their blood. Tibetan natives and other people have shown actual evolutionary adaptions to these conditions, and high-flying supers either share similar traits or possess alternate mechanisms (such as long-term oxygen storage, like whales, or even independent systems for sustaining cells that doesn't require oxygen at all).
Flying supervillains have found it advantageous to build their lairs on mountains. The top ten tallest mountains in the United States are all found in Alaska: Mount McKinley, Mount Saint Elias, Mount Foraker, Mount Bona, Mount Blackburn, Mount Sanford, Mount Fairweather, Mount Hubbard, Mount Bear, and Mount Hunter. Colorado is the big winner for those who prefer the continental United States, with peaks at the 14,000 foot mark throughout the Front Range and Sawatch Range. Such lairs are largely impregnable without serious effort, and to date, no such lairs have been successfully raided by mundane law enforcement. They have been successfully hit by heroic supers, though - you're not the only one who can fly, so beware!
It's been observed that people aren't accustomed to look up when searching for someone. That's usually true, but military and elite law enforcement teams are getting better training these days, so don't count on just floating on the ceiling. What you can count on is getting to hiding places that nobody can effectively search, even if they suspect your presence.
In general the human body cannot be detected on air traffic control radar, military aircraft radar, and the like. Long-range targeting systems will find it a challenge to pick you up. Sophisticated thermal sensors, motion sensors, and anti-missile systems are all built to pick up small, fast-moving objects - that's you, by the way - so you're not totally undetectable while flying.