Matt,
I had seemingly the exact same experience that you had, with my left hip. I'm not sure how far out from my surgery it was, but it could have been about 6 months. I had gone on a long (long for me at the time, meaning about 3.5 miles each way) Sunday morning hike that was steeply uphill on the way out and, obviously, steeply downhill on the way back. When I got home I sat in a hot spa for awhile and then jumped in the cool pool water. When climbing the steps to get out of the pool my left hip squeaked. I couldn't figure out where it came at first from so I got back in the pool and climbed out again. It was my left hip, and it sounded just like a squeaky door. No pain, just squeak.
I found that for the rest of the afternoon I could get it to squeak if I bent over like I was going to pick something up off the floor. I tried to moderate the tone of the squeak so I could play a tune, but it didn't work. When I tried to show people at work the next day it was gone, and it's never come back.
When I asked my surgeon about it he said that the cause is probably not dehydration, but that the lubricating fluid can get squeezed out of the space between the ball and socket by some types of movement, and that it has to work its way back in. Nothing to worry about, unless it persists because it could be an indication of increased friction that can cause the surfaces to wear down faster and potentially shed metal ions.
Mike