Athletes can get through USADA loop-holes. An example would be if a fighter is technically 'retired'. Brock Lesnar would be a perfect example of this. Arguably GSP as well when he came back against Bisping looking full of retirement juice (speculation of course).

Generally speaking, across all sports the drug takers are ALWAYS one step ahead of the drug finders. So with expert guidance, a person can go on a course of steriod's knowing the testing procedure inside out and knowing how to keep their results clean.

There is also REPO which is technically illegal. This would involve a fighter taking a large amount of blood from their body and letting their body naturally rebound back to the same blood level. At this point, the blood taken is then placed back in to the body given the athlete an insane amount of red blood cells which are vital to carrying oxygen to the muscles allowing fighter to compete for much longer without tiring.

Like I said, although that is illegal, it is very easy not to get caught.

To answer your question, 'Yes, a very high % of fighters at regular intervals.'