Undeletable files after kyua test runs

Hi,

I recently stumbled across undeletable files that are generated by kyua test runs,
for example

-rwxr-xr-x  1 root  wheel  0 May  9 13:10 /tmp/kyua.aB4q62/8676/work/fileforaudit

I haven't yet identified the test that generate those files, but it is impossible
to delete them. I have clear_tmp_enable="YES" set in the /etc/rc.conf, but 
on every boot the system argues that these file aren't deletable. 
I tried to 'rm -rf' them by hand but, even this wasn't possible. I have looked for
any extend attributes, but I didn't find any.

Has anyone an idea how this is possible and may how these files can be deleted?

--Gordon