Sexy Sounds

My wife and I went on a cruise last week. We stopped at port on Monday, and most of the guests went on an excursion at the beach in town. I got a little tired, and decided to go back to the ship, while my wife stayed at the beach. I took a nap, and when I woke up, I jerked off silently and cleaned myself up. I was laying on the bed looking at my phone, when I heard a vigorous slapping sound softly penetrating the walls of the stateroom next door. At first I thought it was a guy jerking off, but as it continued, I wasn't sure if I was hearing a man or a woman masturbating. I just knew someone had the same idea I'd had earlier. There were some buzzing noises, which suggested a vibrating dildo was in play. Several hours later, when my wife and I were heading out to dinner, we opened our door, and the door next door opened at about the same time, and I saw an attractive middle aged mom and her adult daughter (20-ish?) coming out, also heading for dinner. We'd seen the duo by themselves several times, and it was clear there was no man staying in their room. We didn't speak to them or anything, but I kept my eye on them the rest of the week, with a constant mental fantasy about which one was masturbating that afternoon. My guess is that it was the mom, but I've pictured each of them doing it. I would never cheat on my wife, especially on a cruise like that, but it's certainly gave me something sexy to think about during the trip. I want to believe she heard me jacking off earlier, got horny from the sound, and did the same.