If you have unplugged a wemo device the wemo app may still have it registered and will look for it. In case the wemo app unexpectedly freezes or becomes unresponsive force close it and then open it again.

Belkin Official Support Understanding The Wemo Mini Smart
Wemo smart plug not detected. Sometimes one device is not detected. Sometimes the other or all devices are not detected. Depending upon its behavior you can tell what state your wemo mini is in and use this article to assist in troubleshooting any issues it is reporting. Your wemo device should be plugged in and connected to the same wi fi where you manage your wemo devices. If so try to restart the app. After resetting them it shows normal.
After the upgrade the app started not detecting my plugs. This will be grayed out in the wemo app. Wemo device not found check if your wemo device is unplugged or removed from the wi fi. Make sure that the wemo device isnt blinking orange see the wifi light status indicator section. While they are easy to use there are many issues like glitches in wemo app losing communications etc. Here are some common issues and troubleshooting of wemo switch mini and insight smart plugs.
Then several days later they are not detected again thsi has been happening with my plugs since the recent firmware upgrade. My device is not detected. Have you ever looked at your wemo mini smart plug f7c063 noticed a light on it and wondered what it means. If you have unplugged a wemo device the wemo app may still have it registered and may detect it. Reinstalling the wemo app usually fixes this. Wemo device not detected or unreachable if youre having issues controlling you wemo device using the official wemo app we encourage you to try the yeti app and see if reliability improves.
Wemo technical support why smart plug show not detected on app my smart plugs show on the app as not detected. The light you see is the wemo mini status light.