My desired username is taken, but I can't find the other profile

I had a Unity account for many years, but didn't actually use it. Now I logged in and the site forced me to pick a username. So I typed „MegaBrutal”, that I prefer to use almost everywhere else. However, I got an error message that I can't choose this username, because it's already taken. It's annoying, but it seems someone else also chose this nickname in the last years and is taking it before me on various platforms, while I'm using it since like 2002. I only got this problem in the last few years. Or it's possible that the other „MegaBrutal” is somehow also me – who knows, maybe I registered another account in the past because I was too dumb to remember that I already have one. So I wanted to find the other „MegaBrutal” to check their profile, I could probably recognize whether it's mine or not, but when I tried to find this member, I got an error that no such account exists. Weird. Is it really taken? If someone else is legitimately using this username, that's fine, I can't do anything about it. But if it's mine or is not actually taken, then I'd like to use it. What can I do to make sure?