readonly mount is not properly dismounted

Hello, forks,

WARNING: /mnt/tmp was not properly dismounted

I think all of us familier with this messages.  However, I am wondering why I sometime get this message on read-only mounted filesystems.  To be more exact, I have some uzipped files and from time to time, I get this warning on these.

Does anyonw know why read-only filesystems get dirty?

Thanks,
Hiro