The only thing I can think of is that you might have accepted a mission. Missions of the form "take 4 Rutile to Gallifrey" will place that much cargo in your hold (marked as not being your property).
Look for the Transactions tab in the left UI panel and that will list any outstanding missions you are on.
If you are sure that you *haven't* accepted any missions then you may be seeing some kind of bug - certainly you are the first person to complain of cargo appearing in your hold at random I have heard of for ~5 months of playing
