THE CORROSION!
I wonder if the corrosion is some sort of beacon for the ship to hone in on across hyperspace? New question; how many interdicted pilots had corrosion damage at the time of interdiction? That is, corrosion damage, so if they'd just recently been carrying a probe and their ship was still damaged from the corrosion?
Because for there to be corrosion, there must be something causing it - is it actually some sort of acid leaking out of the probes, or is it some sort of spatial distortion the probes let out (the mirage effect around them and the bigger ships) - either of which could attract those other ships?
Also, if the aliens are using some sort of biologically-grown ships, then our ships are literally beds of nutrients for them. I wonder if the "corrosion damage" isn't the probes trying to grow stuff out of the petri dish of nutrients that is our ships, and if one of the things they grow isn't some sort of beacon?
I didn't have corrosion resistant cargo racks and got it after only a couple jumps.
I have read in a very old GalNet article when the UAs were new and fancy, that the corrosion has something to do with a defence mechanism of the UAs, gathering materials from their environment to defend themselves/make up for those that have been lost. Something like this. But I can't recall details as it's been such a long time.