Web sites with a Facebook pixel were communicating with local native apps (Instagram, Facebook, maybe WhatsApp) to log (and attribute) incognito web browsing activity.
Write-up: localmess.github.io Story: https://english.elpais.com/technology/2025-06-03/the-covert-method-meta-uses-to-track-mobile-browsing-without-consent-even-in-incognito-mode-or-with-a-vpn.html https://localmess.github.io/