Hope you're running all the up-to-the-minute anti-fingerprinting you can find. For a long while any time I went to YouTube even in a private window they'd ask me to log in under my old Google account that I hadn't used on that computer for years. It was creepy as hell and I'm still not sure if they've stopped because they can no longer associate that account with that computer or if they're just confident enough now not to ask.