Something has appeared at https://fccid.io/IPH-03716/External-Photos/External-Photos-4330211 which looks like a bluetooth versioon of the RD Pod, Garmin’s running dynamics pod. This new gadget certainly has BTLE from the description, but it appears to also have a button. I guess this could be to start/stop recording if the device has memory like the recent bike sensors. Garmin have been notably quiet on running power compared to other vendors, preferring to maintain a calculated field rather than properly commit to power in the watches. This device could see that changing.
It could, of course, be something completely different too.