There have been several news reports about aircraft (light planes and helicopters) flying without visible registration numbers or identifying marks in the USA:
The NYPD referred to the helicopter only as “23”—a reference to the number of police officers killed in the 9/11 terrorist attacks—and initially the aircraft had no registration number painted on its tail or side. source
Cimbolic was beginning to worry that he had overreacted when he noticed, on the same flight-radar Web site, the second plane flying higher in the sky, carving bigger loops above West Baltimore. The Web site reported that this plane was a Cessna 560 Citation V, a small jet. But it showed no tail number, offering no possible trail to Federal Aviation Administration records. This only heightened his curiosity. source
Eventually these aircraft were identified (and in the case of the former, a registration number found). But the general question I have is:
Are non-military aircraft allowed to fly without visible registration numbers (tail numbers) in the United States?
Fine print: I am aware that military planes usually have visible tail numbers, this question is about non-military craft. So "non-military" in this case includes federal agencies and state and local police.