You would have to ask the tournament director, because I guess it could change depending on the venue you are at,. House rules.
I play in a WSOP
satellite bar league, and they have a rule that if you are not at you seat when the first card is dealt to you than your hand is automatically mucked... even in the BB and there were no raises.
One time I was sitting at the table and the server was trying to hand me a beer, but she couldn't reach me so I stood up to reach for it and the TD said my hand was mucked and I wasn't even 2 feet from my seat. He didn't realize what I was doing and I guess he thought I was standing up to talk to a girl and only saw me hug this chick that just arrived and me getting back into my seat, but there was nothing I could do about it. I was pissed because the league would always say if we don't purchase food/drinks than the venues would not let us play there anymore, and this guy just penalized me for trying to get a drink ahaha. I had already made my pfr and everything... I think he was just having a bad day or something.
I play in a home game where if you are in the bathroom or smoking a cig outside than the table will yell to you, " Do you want to stay in or fold it's your turn." So even if you aren't there when the last card is dealt you still have an opportunity to stay in, but usually we do this for people that are in the BB. If someone leaves the table all the time to smoke or whatever we start folding their hands as long as they aren't in the BB/SB. This is more of a social game among friends though, not any serious money or anything.
How far away was the side table you went to? Was it a few feet away from the table or across the room? I guess the best thing to do would be to never get out of your seat if you plan on playing the hand you are dealt. Just wait until you have a hand you are gonna muck anyway and than go get the drink off the side table. I mean if you weren't close enough for people to be like "hey it's your turn, Bob raised." than I can understand them mucking your hand.