The Dredger ReadMe only mentions the probability of finding a Dredger, not a derelict:
When you have troubles finding a deep space dredger: There will always be one in the system you are in when loading a saved game. Just keep flying long enough into the darkness of deep space. After this first system they have a 50% change to be somewhere in the deep space of systems with a technical level of 9 or higher. And last but not least: they can be found in any system when there is a salvaged derelict ship.
I've no clue whether the Dredger/derelict relationship is reciprocal or not; the presence of a derelict implies the presence of a Dredger, but does the presence of a Dredger imply the presence of a derelict?
Maybe I should just stop quoting Eric and await his return to the thread instead
"Actually this is a common misconception... I do *not* in fact have a lot of time on my hands at all! I just have a very very very very bad sense of priorities."
--Dean C Engelhardt
Thanks. On each jump? Or only with a certain probability for each jump?
Anyway, with usually only one derelict I'd say, let the pods live.
The addition chance for a single ship in any system is 10% in anarchies and even lower for higher governments. On top of that is a 2% change of adding a 5-ship group. That would be once every 50 jumps. So generally it would be rare.
The Dredgers itself are also rare and never added in the centre of the space-lane itself. There is a 5% chance of adding them in systems of tech level > 9. Most will only generated by the action of just flying far away from a space-lane or by launching the salvage missile.
There is just one exception: When you also have the bigShips.oxp installed, you will see them on the space-lane quite frequently. But than you asked for the presence of big ships by installing the oxp.