It is your responsibility to be on the bus when it leaves. Even if you ask a BTS representative and he or she indicates that it is okay, it's still on you if you miss the bus. We don't know how long it might take you to go to the bathroom. We also have a ton of other people and things to keep track of, and even though we try our best, we can't promise that we will remember to wait. The best tactic to make sure the bus doesn't leave you is to be on board. Failing that, if you have a friend on board, have them let the driver know to wait. With that said, realize that it's never a good look to make 40 + people wait for you...even for 2 minutes. Think about it, 2 minutes x 40 people...that's 80 minutes. Be considerate!