One of the two MS11-024 hotfixes (KB2491683) is not being detected as missing on XP SP3 (and maybe others). However, it shows up as installed if I manually install it outside of NetChk (running 7.8.1340.0 w/ XML 2.0.0.0090).
This is working as intended. I just compared our data against Windows Update. There are two patches that could be applicable to a machine. One will always show missing for XP SP3. The second one in question, KB2491683 will only show missing if the Windows Fax Cover Page Editor is installed on the system. I did a test with both WSUS and NetChk with this installed on a system and both patches show missing.