Are you sure that plane actually has a PIA ICAO number? Or that on the ground spotters were required to find the new ICAO?
I know nothing about this, but was a rabbit hole to explore.
The tail number for his plane is well known. If you put that N-number into an ICAO conversion script (https://www.avionictools.com/icao.php), you get the same Hex number that you see from adsbexchange.com. From what I can tell, the script is using an algorithm and not doing a DB lookup. So, just based on the tail number, you could figure out the canonical ICAO code for the plane.
The strange thing is that adsbexchange also lists the plane as being part of the LADD (Limiting Aircraft Data Displayed) program (but I don’t know the source of this annotation). AFAICT, LADD limits public display of information, but only for data distributed from the FAA. Crowd sourced data isn’t limited (nor could it be).
In order to address this privacy shortcoming, the PIA program exists. This would change the ICAO for the plane to be disconnected from the tail number. But from the tracking website, it seems like the ICAO number for his plane hasn’t been changed/anonymized(?!?). So either, they haven’t actually gotten the new ICAO number installed (after 10-ish months), or the plane isn’t eligible to be part of the program. PIA requires the plane to only fly domestically and have the right kind of transmitter. If the plane flys internationally, it can’t have an anonymized ICAO number.
See: https://www.faa.gov/air_traffic/technology/equipadsb/privacy
Otherwise, I don’t see how anyone could track this particular plane if it did have a PIA ICAO identifier and you only knew the tail number.
Like I said, I don’t know very much about this, but it is a fascinating rabbit hole. Anyone want to correct where I went wrong?