Well, as I see it, scoopig up lone pieces cargo should count as stealing.
Ideally, in the future (haven't read up on the plans of the devs on that), the owner of the cargo should have some options (possibly through a menu on a base)
1) Allow salvage: at the location of the cargo in the system will be marked with some kind of a signal source, like "salvage signal source detected". If it is near a starport, it's marked as salvagable (perhaps a colour change from white to turquise on the radar).
When scooped up and sold, the original owner get's 50% of the proceeds, but has to fetch them at the base the cargo was sold to.
Illicit people might elect to sell those goods on the black market though for their own profit. Thus it's a bit of a gamble on the honesty of people, as it won't be marked as stolen in their holds.
2) Finder's keeper: The original owner renounces ownership, thus it's free for all to pick up and sell it legally.
3) Retrieval mission: The original owner starts a mission on the bulletin board on the system he's in and pays 60% of the local value as a finder's fee. One pilot can accept it, and bring the cargo to to said base. Via the Bulletin board, the Fidner dumps the cargo there, recieves his reward, and the original owner an claim it via the bulletin board aswell.
4) Retain onwership, retrieve it himself/herself: Noone is allowed to scoop it up, but after a certain time, local authorities confiscate it and fine the original owner, plus bill him the retrieval costs (minus the local sell value of the confiscated goods).
Pirates and smugglers though can always elect to scoop up any cargo and sell it as they normally would.
Of course, that would need for the cargo to be a bit more persistent to work.