Very strange. N428SS flight plan for Denver.
ICAO hex code search returns nothing
http://milradiocomms.com/search_mil_hexcodes.php?type_of_search=hex_search&search_hexcode=A51AF8
FAA has it registered as a reserved, not active, N number.
https://registry.faa.gov/aircraftinquiry/Search/NNumberResult
https://flightaware.com/live/flight/N428SS
https://www.northwestflyers.com/