Rashi (on Pasuk 9), Ramban, Rashbam (both citing Bava Kama, 94b) and Targum Yonasan: It is referring to a Shomer Chinam, whom the Torah renders not liable for theft (and loss).

Ramban and Seforno: We know that from the fact that the Torah changes from money or vessels (which people tend to safe-keep for frees, to animals, which they generally place under in the charge of a shepherd, whom they have to pay.

Rashbam: Because, since the former is safekeeping inanimate objects, ¹ the owner knows that the Shomer will place them in his house together with his own belongings, which he guards with care; whereas the latter, he places in the field, ² and he expects him to keep a watchful eye on them.

Rashi: The Pasuk is referring to the Shomer's claim, not to what actually happened.

Ramban: The Pasuk is indeed speaking where the article is stolen, and it then draws a distinction between whether the Ganav is found or not. ¹

Moshav Zekenim: The owner must say 'here, guard this for me.' If he said only 'keep your eyes on it', he is not a Shomer, and he is exempt for [passive] negligence. If he gave it to another Shomer, and we know that Ones occurred, he is exempt; the Halachah does not follow Rava, who says that the owner can say 'I do not want my item in another's hands.' ¹ If one deposited with a woman in front of her husband, he is liable, as if it was deposited with him.