Bathroom cheating happens, but I wouldn't have thought it to happen at such high levels of play. (Though, logically, it probably would as professionals cheat from everything from bicycling to baseball.)
When I used to play in high school, there were a few cases of cheating. In smaller "meets", there was a time limit of 40 minutes per player - sacrificing 1/4th of your time to make 2 moves (max, not like you're going to get away with 3+ bathroom breaks) wouldn't be worth it. In larger tournaments, where time limits were raised, cheating might be easier. However, you'd still probably be limited to 1 or 2 'cheating sessions' at great expense to time, and at the risk of being caught and disqualified.