To be traced or tracked (e.g., the trail of a criminal) — an idiomatic expression meaning that traces or leads are found.
警察が犯人の足がついたため逮捕に至った。
Because the police traced the culprit, they were able to make an arrest.
不正の足がつくと、会社の信用に大きな打撃となる。
If misconduct is traced, it deals a serious blow to the company's reputation.
彼の足がついたのは防犯カメラの映像だった。
What traced him was footage from a security camera.
Idiomatic/archaic: (colloquially) for a woman to be found to have a male lover — an older expression meaning the relationship is discovered.
昔の小説では、女中の足がつくという表現が使われることがある。
In older novels you sometimes see the expression that a maid's affair has been discovered.