Unused Index Entries

Does anyone know what are unused index entries. Whenever I run chkdsk on my
hard drive which has NTFS on it, I always get an message (bootex.txt)
similar to the following: -

Cleaning up 3 unused index entries from index $SII of file 0x9.

Can someone guide me as to what is wrong and why should I receive this
error? The entries however, do decrease with more chkdsk scans. Anyone with
some useful information.....