Another possibility is, when in surgery, don't they hook you up to electrolytes or saline solution? That could cause you to retain water, then when you get back home, and start drinking water and eating a regular diet, your body releases the extra water weight...?
I am not a doctor (nor do I play one on TV). However, when I had a fatty tumor removed (it was a good pound and a half), I retained water and had gained weight at the following week's WW weigh-in, but lost it after a brief time.