A shomer chinam’s (unpaid watchman) type of shemira depends on the object (Rambam, She’eilah 4: 2-3). The gemara (Bava Metzia 42a) says that money must be stored in the ground. Yet, says the gemara, if one received money on Friday, when he lacks the time to bury the money, he is not required to do so.
The Rosh (Bava Metzia 3:21) cites the Ri Abartzaloni, who says that one must put money in the ground only in a time when thieves are prevalent, but not in "our days." The Rosh brings support from the Yerushalmi that says that it is enough for a shomer to put the item where the shomer places his own things. The Ramban qualifies that this is only if he puts things in normal locations. The Rambam (ibid.:4) does not mention the Ri Abartzaloni’s idea, prompting the Beit Yosef to assume he disagrees. The Maggid Mishneh says that the Rambam agrees that one gives an object to be watched based on the assumptions of his time and place.
The Netivot Hamishpat (291:24) says that a shomer chinam is not required to act on the shemira but just to put the object in a safe place. He brings two applications to this distinction. The Machane Ephrayim (Shomrim 38) says that a watchman, during the time the object is under his watch, is not considered the owner’s worker. This makes a difference regarding the law of shemira b’ba’alim, that if one is working for the shomer at the time he started watching, the normal obligations do not apply. This approach also justifies the Maggid Mishneh (Sh’eilah 7:11) that a shomer chinam cannot back out of his responsibility to watch, as opposed to a shomer sachar (paid watchman) who can, because any worker has to be able to stop working.
Regarding a shomer sachar’s obligation if the object is stolen or lost, Tosafot (Bava Kama 57a) asks why he is exempt only if there was an armed robber and not if a particularly talented robber steals it. They answer that we learn from the p’sukim that the Torah obligated a shomer sachar when there is a robbery except in a defined case which is under the category of shvuya. Apparently, they posit that a shomer sachar is obligated for the action(s) of watching, and we say that if the object was stolen or lost we consider that there was not proper watching. Only when it was taken by force do we not attribute the loss to the actions of the shomer. According to this approach, there is a machloket as to when an oness (extenuating circumstance) occurred to the watchman (e.g., he became suddenly sick), not the object. The Nimukei Yosef exempts him because he was unable to watch; the Rashba obligates him if the object was stolen. Tosafot elsewhere (Bava Metzia 42a) says that regarding any oness, even one that resulted in regular robbery, a shomer sachar is exempt. The Shulchan Aruch (CM 303:2) and Netivot Hamishpat (ad loc.:2) accept the first opinion; the Shach (ad loc.:4) and Gra hold like the second Tosafot.