The logger was obviouslly there, it was deliberatly collecting the SSIDs and MAC addresses.
Possibly a debug option to log the whole packets added during development, and it was accidently left on in production.
Or the whole packet was always logged, a second process would then skim just extracting the SSID/MAC (correlating with GPS), and another process was deleting the raw logs. That third process failed.
A few big drives in teh data collection devices, and possibly nobody noticed where filling up a little too quickly.