Some time ago (like years ago, now) the default user was admin and the the default password was password. I suppose it's possible you go a router programmed from then. I also thought there was a time when the user name was admin, but you were asked to set your own password for the router when activating your account. You're right though that most recently, they put on a sticker with the router password and wifi password.
If it is the old admin/password user name and password, be sure to change it. You'll need to set up your own wifi password as well.
Is there a sticker with serial number.
Older routers had been setup with the "password" value. Installers use to change the password to "password1" At some point they started setting it to the serial number of the router and later a unique password was adde to the sticker.
You should be able to see your current password on this site when you look at the my verizon page.
If you have Fios TV you can look up your password via the STB.
You can reset and it will either reset the password to nothing and ask you to assign a password on login, or set it to the sticker value one of the serial number or to "password'. THe reset button is a recessed opening on the back of the router.