Commentary for Bava Metzia 161:21

Guard for me and I will guard for you [gets the law of] a paid watchman: And he is obligated if it is stolen.

But why [should be be obligated]? it is [a case of] watching with the owner: The owner of this object that was stolen was in the employ of the watchman, for also he [the guard] has a guard to him. And it is written: "If the owner is with him he shall not pay" (Shemos 22:14). And we expound later on "with him" [as] "in his employ". And even though this is written by a borrower, later we expound this also on every type of guard in Chapter "The Borrower" (Later, 95a)

And I will guard for you tomorrow: that he is not in his employ now.