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.
