On the planefag notable lb. I found this, older but seems tha 00-9001 reg has been topic of discussion elsewhere before. Seems to be used by State for ‘black’ ops. Search FEST on the second link for the units that do FEST operations. It’s a call sign directory. Keith was not listed, but could be newer.

Here was a user post, first link:

One or two days ago, I read that Flightradar24.com had added a new feeder in Washington DC. So, on Sunday May 08, 2011 at 19.50 GMT I thought it would be nice to try to see which kind of traffic crosses the airspace in the DC area. As soon as I moved the cursor on the East Coast of the CONUS I noticed an aircraft showing the “No Callsign” label. I clicked on it and what I saw really struck me:

The aircraft broadcasting its position, serial, speed, altitute, etc. to everyone was the famous C-32B serialled 00-9001 belonging to the 227th SOF based at McGuire Air Force Base, N.J. This is considered a “black” USAF plane as it is operated by the Department of Homeland Security and US Foreign Emergency Support Team (FEST) and it usually appears in the most unusual locations all around the world. Aircraft spotters have been trying to track its movements and they often report this and few other C-32Bs (B757s) of the Air Force Special Operations Command using bogus serial numbers and always changing Hex codes. Don’t you believe it is somehow weird that such elusive aircraft, deploying US teams in response to terrorist attacks or (as someone speculated) to transport prisoners, was transmitting full ADS-B over the US?

https://theaviationist.com/2011/05/08/black-c32/#disqus_thread

Call sign link:

http://www.udxf.nl/MCL.pdf