I have noticed 2 devices in my logs that are chatting but not joined as devices. I added them to my hub and gave them a generic Xiaomi DTH to try and identuify them. I know MAC addresses will give you a manufacturer ID so you can at least have an educated guess as to the device in question; do device broadcast strings give you any clue as to what might be the type/manu of the device?
It’s obvious I have 2 devices with batteries left in somewhere 
One of them reports a varying inside temp so I know it’s in the house at least, Ha, big help 
| test | Xiaomi Temperature Humidity Sensor | Home V2 | Home V2 Hub | 00000D6F00042B84D1 | E42B | ONLINE | Cloud | 9 minutes ago |
|---|---|---|---|---|---|---|---|---|
| test2 | Xiaomi Motion Sensor | Home V2 | Home V2 Hub | 0000158D0001137A8E | ED69 | ONLINE | Cloud | 3 minutes ago |