Can long flights cause weight gain, i.e., bloat/water retention?

Been maintaining fro 2 years fairly easily. Just got back from a 9 day vacation in Hawaii. I never eat all that much differently on vacation and this was no exception, i.e., I pretty much follow my usual eating pattern and if I veer off, it's only rarely - example: I ate two sushi rolls pretty packed with good stuff. I stayed pretty much on target, otherwise. It was incredibly and unusually hot for this time of year where we were (95+ degrees and very humid), so the activities that I wanted to do (hiking, etc.) were minimal; I was far less active than I am on most trips, and far less than I am at home. More sitting around than doing anything else.

Factor in that I was ill for 2.5 weeks before we went and instead of working out 5 days a week, had to be pretty much a couch slug. I was still sort of finishing up that bout of bronchial infection as we left for Hawaii. So .... It will be almost a month before I am back to my exercise routine ... and I miss it/feel it!

I got back early this AM after two red eye flights back, a total of 11 hours in the air. (Same amount on the way there.) When I stepped on the scale, I was up 4 lbs. That is very unusual for me. I never let myself get beyond a 2 pound range.

So my questions are: do long flights do this to you, where I'd retain water even if I did my best to drink a lot of water? And could eating every meal in restaurants for 9 days add to the water retention, not to mention that kind of heat? I always tell them "no salt" when ordering, but sometimes it's already in the prep before it gets to my plate, i.e., had a veggie patty with breakfast and could taste the salt in it!
Love to hear your thoughts! My game plan is to drink a lot of water the next few days and keep very much to my normal calories.