According to the File System Hierarchy Standard, the lost+found directory is used for:

According to the File System Hierarchy Standard, the lost+found directory is used for:

A.
files with unknown owners

B.
missing device files found by/proc

C.
unknown binary files found by find

D.
unlinked files found by fsck

Explanation:
See http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/lostfound.html