C. F. Keil and Franz Delitzsch on Leviticus 5:2-3

Lev 5:2-3

The second was, if any one had touched the carcase of an unclean beast, or cattle, or creeping thing, or the uncleanness of a man of any kind whatever (“with regard to all his uncleanness, with which he defiles himself,” i.e., any kind of defilement to which a man is exposed), and “it is hidden from him,” sc., the uncleanness or defilement; that is to say, if he had unconsciously defiled himself by touching unclean objects, and had consequently neglected the purification prescribed for such cases. In this case, if he found it out afterwards, he had contracted guilt which needed expiation.