Sometimes it shows up as a narrow void, or reverse spoke. I have seen it from ships off San Diego, Calif, and Southern Nevada. To verify it's a ground source, turn the plane a few degrees.
Mil. a/c used to use airliners as practice targets - until TCAS came along and showed the games they were playing. Even after that, America West was seeing lots of pilot squawks for phanom targets over Southern Nevada. It was apparently from nifty ground radars that could make your TCAS look like you had four bogeys approaching from four directions. After a technical management meeting, an AMW pilot, who was USAF Reserve, got in touch and told them to knock that crap off. That pretty much ended the unverified removals of TCAS processors.
GB