disk image mounts automatically

Since a couple of days I am experiencing a mysterious problem: a disk image (.dmg) file keeps mounting automatically under certain circumstances.

This is what happens:

- when I reboot my Mac

or

- whenever I choose 'Print' in an application and then select a print option or choose a printer from the printer popuplist

the image mounts and the windows displaying the contents shows in the Finder.

The image is a MacUpdate Promo image of 2009. It is read-only and it is stored on a network drive (a USB disk attached to the Airport Extreme). I can unmount the image, and the network drive, no problem. No files in use. But when I choose Print etc , it appears again!

Now, this image has been there for almost a year, and the problem started only two weeks ago, when I searched the disk for a piece of software (which I didn't find, so I never installed anything off the disk image).

I'm running MacOSX 10.6.8 on a MacBook Pro 15" (2010).

I am not aware of any preference setting that causes this. The .dmg file is not in the list of login items.

Anyone an idea what's causing this?

HE