Ok so time for a bit of out of the box thinking. The great MB has said that you are guaranteed to find them if you are in the right location. FD are presumably pretty good at Maths so they will know that even with 1000 CMDRs scouring every landable moon at POI height would take essentially forever. Secondly Kerrash mentioned that he thought that it would be visible from orbitalish height - although he may have no expert knowledge so a caveat there.
However if both these possibilities are correct then we have not been to the location yet even in orbital cruise. Given the number of people involved this seems unlikely.
theory: the dashed things are on a planet immune to the powers of the advanced discovery scanner. Trying a bit of old school parralax stuff in (most obviously) Merope might be worth a shot?
Had a very similar thought, the UAs are cloaking something in Merope, searched the inner system from above and found nothing - didn't check outer system though.