Lucy Mack Smith, in her biographical sketches, says Joseph possessed "keys" to "discern" invisible things.
Lucy Mack Smith, Biographical Sketches of Joseph Smith the Prophet, and His Progenitors for Many Generations (Liverpool, S.W. Richards, 1853) 91–92, 106, The Joseph Smith Papers website, accessed August 11, 2021
A short time before the house was completed, a man, by the name of Josiah Stoal, came from Chenango county, New York, with the view of getting Joseph to assist him in digging for a silver mine. He came for Joseph on account of having heard that he possessed certain keys, by which he could discern things invisible to the natural eye.
. . .The thing which spoke of it had that Joseph termed a Key was indeed nothing more nor less than the a urim and Thummim by which the angel manifested those things to <him> that were shown him in vision by the which also he could at any time ascertain the approach of danger Either to himself or the record and for this cause he kept these things constantly about his person
After the recovery of the plates, Lucy refers to the Urim & Thummim as a "key." Josiah Stowell's hiring of Joseph took place before the recovery of the plates. This means that the "keys" referred to in this case are most likely Joseph's seer stones.