Try contacting local small airports for information. Also, flight plans are apparently public record and available upon request.

Flight plans are not required if you're just out flying VFR. Someone who doesn't like ADS-B probably also doesn't call uncle sam and tell him where he plans to fly.

Do you have your own ADS-B receiver or how do you know it doesn't have ADS-B?

You can track them with sites like FlightAware or FlightRadar24. Just zoom in to where your house is. Click on a plane. If it's not on the map, it likely isn't broadcasting ADSB

https://www.flightaware.com/

https://www.flightradar24.com

They aren't visible on these sites, first thing I checked. Maybe they've requested delisting but I think they're just not broadcasting—specifically to avoid consequences of their reckless flying.

As others have noted these commercial services don't show every plane, the owners of planes may request to hide their planes there, that's why I asked how you can be sure.

Other sites like https://globe.adsbexchange.com or https://globe.adsb.fi don't filter aircrafts, they even show military planes (well, if they have ADS-B enabled). Though they don't have coverage above oceans, as satellite ads-b is expensive. (These sites look nearly the same, as they both use tar1090 as the webui, but they may have different coverage)

These sites, may sometimes have less coverage, as the sites you mentioned are way more popular. But if your area has coverage by these sites, you can be sure that you'll see every plane if it has ADS-B.

You could even replay, if you know the time and date when the plane passed, by appending ?replay to the URL.

e.g. https://globe.adsbexchange.com/?replay

Thanks, this is great info!

Laser pointers mounted on auto-aiming platforms, connected to the ADS-B servers.. no ADS-B data? Aim, fire!

No. Big crime. FAA calls FBI. FBI calls local PD. Men in black. Door kicked in. Handcuffs. General bad vibes.

ADSBExchange is more complete (at least in my area).

These services rely on volunteers setting up receivers and feeding them data. It is entirely possible that there is just no local coverage that would receive the low flying aircraft.

A 30usd rtl-sdr dongle is enough to receive local ads-b signals, if one wants to test it.

Have you tried a super long telephoto lens? If you got like 100 photos in a row you might be able to use some kind of software to blend them and infer the number.

It's only been a couple times a week on random days and times; I'd basically have to camp on my roof to catch them. And they're going full tilt (at least 90 knots) so there's only 5-10 seconds of opportunity. I do have a pretty good long lens on my digital camera, maybe I'll set it up and have it by the door.

Start writing down the exact time of the events, that way you can piece it together without having to sprint outside with binoculars.

When I started paying attention to airspace as part of my 107 drone cert, the amount of extreme bullshit flight behavior out of the two local small fields genuinely surprised me. It makes me think it's no wonder general aviation has so many accidents.

(The fields are San Martin and Watsonville in the Bay Area, FWIW.)

do you live near an airport?

Nope.

4 days late on this but - if an airfcraft is in a sparsely populated area then it is allowed to be below 500' AGL so long as it is 500' away from "person, vessel, vehicle, or structure." This rule is suspended if the aircraft is in the process of taking off or landing (includes "flying the pattern"). I don't know where you live, so it may not apply to you. If you're in a rural-ish area, the person could be taking off from their own land or an airport you're not aware of.