Watch 7 ultra, all through the night and several times a day my watch will vibrate once, followed by 3 vibrates. No apps notifications, seemingly for no reason at all.
I also experience phantom vibrations on my Watch7 44mm. I originally chocked that up as a random bug in the Watch7, but since then I acquired a Watch5 Pro that exhibits the same problem AFTER having received the recent 1UI6 update. Watch7 came with the update. The Watch5 did not appear to exhibit the phantom/ghost vibrations until it received the recent update. My conclusion is that the vibrations are Wear OS5 related. Other people will no doubt feel there is another explanation, but even though they are a pain in the backside, I tend to just ignore them at this point as much as I can.
