From what I have seen, the answer is maybe/maybe not. ATC radars often have ways to filter out weak returns to remove clutter and reduce controller work load. I remember having a controller demonstrate to me once, he could adust it to show only those returns that had active transponders or show any primary returns (skin paints). At the skin paint setting, he pointed out a bunch of clutter, a flock of birds, a few hang gliders, and a formation of 4 Air National Guard Fighters. At the highest level of filteration only the lead Fighter showed up since only he had his trasponder on. This was some years ago so I imagine more modern systems have a more refined selection of filters. Having done a fair amount of large CRW, I have occasionally seen the aircraft I had just exited circling the formation at a safe distance. I was told that this allowed ATC to more easily moniter the formations approximate position and altitude using the aircrafts transponder. Skin paints usually do not give altitude information. As for as free fall goes, I would think it would provide a more difficult target. But not immpossible.
