The laws of guardians or watchmen are more complicated because there are four types: borrowers, renters, unpaid custodians and paid custodians. Basically, a borrower says, “Can I use your car while you’re away?” A renter says, “I’ll give you fifty bucks to use your car while you’re away.” An unpaid custodian was approached with “Will you watch my car while I’m away?” and a paid custodian was told, “I’ll give you fifty buck to watch my car while I’m away.” The laws vary as each type of guardian is responsible for a different set of possible mishaps. (Possible mishaps include theft, loss, accident and negligence.) Here we are speaking of the shomer chinam, the unpaid watchman.
If a person is watching someone else’s property as a favor and that property is stolen, the guardian must go to court and swear an oath that it was really stolen and that he’s not secretly squatting on it. If it turns out he’s lying, he pays back double, just like any other thief. (In Mitzvah #30, we spoke about the importance of avoiding needless oaths. This oath is not needless. It’s a perfect example of an appropriate and necessary oath.)
This mitzvah is incumbent upon the courts. In the Talmud, it is discussed in the tractates of Baba Kama (ninth chapter), Baba Metzia (third chapter) and Shevuos (eighth chapter). In the Shulchan Aruch, it is found in Choshen Mishpat 291. It is #242 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.