I think they forgot to turn it off as the HEX code changed a week or so ago.
ZZ330 was 4065C1 but I picked it up on the 18th May as 43C6F3, a UK military HEX code.
They have been doing some flights out of Brize, up to Aberdeen, across the Highlands to Stornoway then turning to head South again down the West Coast, once landing at Prestwick.