(22:9) "no one seeing": Scripture here speaks of (the absence of) witnesses. "no one seeing, (10) the oath of the L–rd shall be between the two of them (that he did not send his hand against the deposit of his neighbor.") But if there are witnesses (that he is speaking the truth), the watcher is exempt from all (payment).
The Mekhilta Explains No One Seeing Means No Witnesses Were Present
Mekhilta Tractate Nezikin 16:6