I’ve seen this a few times, with my TAS605A.
This merges TAS (Mode A/C/S) and ADS-B OUT targets. It uses the 24 bit ID, which exists in both Mode S, and in ADS-B OUT (when emitted by a Mode S TXP).
The rationale behind this is that, in Europe, you cannot emit ADS-B OUT outside of a Mode S TXP context.
But of course you can if you have say a SkyEcho 2! In the SE2 you can configure any 24 bit ID and any tail number, and you can suction-cup-attach it to a window in an aircraft with a Mode A/C TXP.
In the UK, keeping old Mode C TXPs is very popular, due to the CAA’s aggressive infringing pilot persecution policy.
I wonder if there is another explanation however. What if you had an SE2 configured with a different 24 bit ID from the one in the Mode S TXP. I’ve just checked one aircraft which showed up as two this morning (“both” got fairly close to me, near SFD, about 3000ft) and it shows up on Mode S but ADSBexchange shows nothing for its tail number.
Surprising nobody else has seen this, since it is reportedly not that uncommon.
Next time I see it I will get the time so one can look at tracking sites afterwards.
Another one today. This is not rare…
It was around 0913Z today. I don’t know how to check a tracking site for “any” aircraft around a location and timeframe. Their UIs are pretty weird. It is not impossible there was a formation flight there.
From EXIF:
lat 50, 46’ 30.489240"
long 0, 0’ 25.921800"
alt 1288 m
Thank you 
You can on ADSB Exchange
https://globe.adsbexchange.com/
Press the replay button and set time and date – it plays back the data for that time
Amazing! I would not have found that feature…
He’s the one heading north. And sure enough, 400ft below. But just one aircraft.
Unless there were two and the other was Mode C only. That’s unlikely. This one is based at EGKA and always flying around that area. I’ve had multiple returns from them.
Strange return on FR24 now for G-BOZV, two aircraft showing with one saying broken or misconfigured transponder. One icon is a jet the other is a single engine aircraft.
