My wife has a Dexcom too (G6). You might also be interested in NightScout openHAB Integration (you can set up Nightscout as a follower to CLARITY so you don’t have to use xDrip or Spike to populate it I think). And I can confirm, at least with the G4 (I haven’t tested this in a long time) that the receiver shows up really well using reelyActive and other BT scanners (there is a not yet accepted BT binding that would probably work too) as Dexcom didn’t bother to spoof the BT address for privacy.
I should see how it works with the G6. I can probably inspect the broadcasts and see they are coming from a G6 sensor and don’t even need to worry about the fact that a new sensor needs to be inserted every 10 days (longer if you are using xDrip) potentially changing the broadcast source address.
As for the Apple Watch, you will be able to see that an Apple watch is present, but not that a specific watch is present. Apple is pretty good about privacy in this respect so the watch spoofs it’s BT address, changing it every few minutes. There might be something in the message that can uniquely identify the watch though, I haven’t looked. My wife quit her iPhone and Apple Watch when she decided to move to xDrip and Nightscout and I prefer Android.
NOTE: While Dexcom does offer an API to get the readings from CLARITY, they delay the data by 30 minutes making it almost useless for driving home automation. You can see my experiments there at OAuth2 using just OH Rules and myopenhab.org.