So yea ive found this out the other day and pretty much got it all down. to find this location, when you start off on the free explore, just go left and keep going through all teh tunnels and such and youll eventually make it to Skywatch. enjoy the grind my friends.
Alright, I really didn't want to share this, because I've found that more and more people are horning in on my area, but here it is. When you spawn in free roam, go to the right and up the stairs. Follow the tunnel to the end and you should come out in a large clearing. Go to the right and get on top of the highest building. There are/were three loot chests there. In addition to the chests, the area is also the spawn of three different Elite Fallen enemies, each giving a median of about 25 Glimmer each. They also drop Ether Seeds. I recommend bringing a Scout Rifle and just perching on the building. It's what I do. Just keep flipping around and sniping enemies across the clearing. Using Ether Seeds, you can easily make 1000 Glimmer in half an hour.