What is going on?

Can anyone tell me why FlightAware ‘near me’ flights show unknown flights all labled as B then numbers followed by 1 or 2 letters eg. B1300QQ.
Some observations:
Appears to only be in New Zealand.
Often start near airfields but positions do not coincide with tracks one normally sees for aircraft.
Seem to have groundspeeds of single figures throught to 260 kts.
Flights of 2 minutes to 4 hrs plus.
Tracks often follow light aircraft including helicopters but not larger/faster commerial traffic.
Flight tracks can start or finish offshore.
All show as small jet icons.
TSV data groups do not show altitude and are a minute or more apart compared with around 4 lines per minute for other planes.

For the record, I install ADS-B systems in gliders which is why I check the TSV data for first flights.