The confirmation by MB that some instances of meta-alloys/barnacles/whatever (ie something we want to find no matter what it turns out to be) are hand placed is interesting. I did some maths many pages ago that demonstrated the total futility of attempting to do a ship-based survey of just the landable bodies in Merope, let alone doing it by SRV. Therefore it seems to me that if you were hand placing these things you would put them in places you thought people were actually going to look. There would be no point just placing it in the middle of some big, anonymous flat area of a moon because the likelihood that somebody would randomly land there and stumble across it would be so low as to make the whole exercise pointless. That is unless you hand placed thousands upon thousands of them which would seem like a tremendous waste of time to begin with.
For the last few days I've been flying around in the Pleiades and landing on any location that attracted my attention from orbit, be it an unusual crater or oddly coloured valley. So far nothing. Also, the sheer size of some of these worlds makes this method infuriating at times. I approached the largest crater on (I think) Taygeta 4 because it was unusually big, then saw that on the rim of the crater was a small area of strange colouration that looked intriguing. By the time I got down there that speck of strange colour revealed itself to be so enormous that you could have probably fit the continental US into it with room to spare. Most of the time even tiny features seen from orbit are so colossal as to be unsearchable within any reasonable time period.
I'll keep at it obviously, but I feel like we're waiting for Palin or some other clue to point us in the right direction and that right now even the hand placed instances are beyond our reach if only because it's like searching for one particular grain of sand on a beach.
With regard to his comment about nebulae being the logical place: I think this is more of a gameplay thing than a scientific reason. If your plan is to have certain areas of the galaxy be more likely to contain these things than anywhere else, the obvious choice is nebulae right? What other distinct structures on this scale does the galaxy have? None come to mind. There's basically no other choice.