Mishnayos Bava Kamma Perek 10 Mishnah 8

חייב באחריותו – for once he stole it, it exists in his domain, and the restoration that he made is not a restoration (without notifying the owner of such).

ואם לא ידעו בו וכו' – This is how our Mishnah is reconciled, he is responsible for it, whether he counted them or not. When is this case? When they (i.e., the owners) did not know of its theft and its restoration. But if they knew of the theft and counted the sheep and found it (i.e., the number) to be complete, he is exempt from paying. And this is its explanation: if the lamb died or was stolen after the thief returned it, the thief is responsible for it – whether the owners had counted the sheep and it is complete, or whether they had not counted the sheep. When is this the case? When the owners did not know about the theft of the lamb, for any of the sheep that regularly go outside, he must be careful with it even more, and this is the case, for since the owners did not know about its theft or that it was accustomed to leave, and they were not careful with it. The thief is liable to pay as the sheep were lost because of his indirect effect. But, if the owners knew about the theft of the lamb, and afterwards counted the sheep and found it to be complete, as the stolen lamb had been returned. If so, they already know that there is one sheep that regularly leaves outside, and they have to be careful with it. But if they were not careful, they would lose, and the thief is exempt from having to pay.