Did it actually fly with a covered reg and no interim reg added? Parking should be okay, flying not. Some clandestine VIP-ships these days tend to have their regs intentionally shielded by their oversized engine-covers while parking. Aside from less vanity regs and colour schemes exotic overseas countries (like the cayman islands) seem to become more popular to have your a/c registered at. (not only due to lower cost) If you want to remain low profile this is way less spectacular than flying without anything. Guess they just sold it to somebody. Or might it be grounded for some reason?