Helicopter pilots flying tours over Hanauma saw it.
Workers at the bay wondered about it, too, after the aircraft buzzed over and over and over again.
"I saw the plane coming in low from the Diamond Head direction, across the Makapuu Lookout. It paralleled the Waimanalo Range and came back in over the Koolaus," said John Aimo.
"It had no markings. It looked like a "Larry Ellison" jet. It was a private jet with no markings. It paralleled Kalanianaole Highway, both times, one time over the sea and once over Makai,” Aimo said.
Steven Motz said initially he didnt think too much about it.
He first noticed it at about 9:30 a.m. above Kamehame Ridge.
But after it buzzed overhead four times he managed to snap a few photos as it flew over Portlock.
"It was very low and not anywhere near an airport. It was far too low for a jet that big. It was so low, I saw it had a blank side so I initially thought it was a drone, but who knows?” said Motz.
The East Honolulu scenic skies are unusually busy with everything from hangliders,to parasailors, to helicopters, and other general aviation aircraft. But this unidentified flying object caught the attention of more a few people who wondered what it was up to, and whether the FAA knew anything about its unusual flight patterns.
“My clients I was visiting saw it as well, and were wondering how suspicious it was. What was that plane doing so low?" said Motz.
The FAA did confirm it is testing a new system of navigational aids.
It started Tuesday and may continue through the week.
The agency said it has a variety of aircraft with onboard computer systems that measure navigational signals to make sure they are in proper working order.
On Wednesday, a FAA spokesman said it was flight checking procedures for the Honolulu International Airport.
The FAA confirmed it does have Lear jets in its fleet, so it is likely what area residents and motorists have reported seeing.