Teeth or tooth?

[belly_ttt]

Sarah turned out to be a fed member who acted as a bait so that the FBI could trace her through a so- called "positioning chip" implanted to her teeth.

1) Am I right to add so- called here?
2) Teeth or tooth?

Thanks in advance

[Anglika]

Why would you call it "so-called" - it clearly is a positioning device since the FBI used it to trace her.

And presumably the device is very small, so is only implanted in a tooth.

ALSO ...who acted as bait...