There is a bug in iOS 11 (has ben reported by others too) when using wOS 3 (not sure if it affects wOS 4) that can cause your Watch to lose it's connection to your iPhone. It will not show up under BT connection and there is no way to reconnect it without un-paring (losing all data since it became disconnected)/re-paring. When this happens things also get really wonky. For me yesterday I first noticed that my iPhone didn't unlock my Watch. While home it gave no indication of not being connected. When I left home to the gym noticed the "not Connected" red icon. When I got back home I tried every combination of restarting/rebooting/switch toggling/swearing etc. I could get a partial connection (see pic with weird data & also took some shots from my watch but were lost in re-pair) of some sorts but it didn't show up under BT Settings or reliably transfer data. Luckily I had enough time in the day to get all my Stand (5 hours lost), Move (again) & Exercise (again).