I would love to build a simple setup where the onion2 can run on a battery and be placed in a park or similar. here it should log all "devices" that it sees when scanning for wifi/bt devices in the surrounding. I would like to have a bunch of these setups in the same park/plaza and use the logged data from all of them combined to do a sort of movement pattern analysis for the guests.

I have the exact same problem.
I tried running device-client -v to see what was going on and it seems like my device does not event have a key.

Starting device client
Starting listening...
Sending request to 'ds.onion.io/qwqw/listen?key=qwqw'
Write_callback: received '23' bytes, time: 'Wed Jul 12 15:14:12 2017
'

Write_callback: valid json: '{"error":"auth failed"}'
error : "auth failed"

Completed GET
Connection lost!
Restarting connection (restart #102) ...

RESPONSE THREAD!
Sending POST to url: '', post data: '{"error":"Could not find 'command' object!
"}'

POST Sent! (url: '')
Error: Error

Closing RESPONSE THREAD
Starting device client

I would love to build a simple setup where the onion2 can run on a battery and be placed in a park or similar. here it should log all "devices" that it sees when scanning for wifi/bt devices in the surrounding. I would like to have a bunch of these setups in the same park/plaza and use the logged data from all of them combined to do a sort of movement pattern analysis for the guests.