If you haven't activated it and you haven't given your BTID details to anyone you know, then I'd recommend changing your BTID password ASAP. The only way the Xbox Game Pass can be activated, is through MyBT. So if it wasn't you or anyone you know, then an unknown third party has gotten access to your MyBT login details. If you use the same password elsewhere, I'd also recommend changing that.
To get your Xbox Game Pass ceased from BT and refunded, you can either await a Forum Mod seeing this, or call the Helpdesk on 0330 1234 150 🙂
My details have been changed. but I thought the 2 step log in stopped third party access?