Well, the first thing that would rule out "kids" for me is not what's there, but what's not there. No beer cans/bottles, no food wrappers, etc. Sight appears very clean. If it was a party palace, that wouldn't be the case.
Occoult useage is always a possibility I suppose. Are there any candle nubs, dripped wax, incense sticks, etc in or around the structure?
The holes could also be the clue...what do you mean by holes? How deep?
How far apart? Is the canopy open overhead? I ask, because there are those who attempt to grow pot out in the preserves. But because the soil is not conducive to good growth for that plant, they will spend weeks getting it ready. The first step is digging the holes. Then they will backpack the right soil mixture in to fill and plant. Then comes the chicken wire to keep out the rabbits. Have seen this ALL over the preserves in various stages. Usually, they get sloppy and leave the empty soil bags near the parking lot or trail head. If that were the case, the structure might be a rally point for them, place to meet, rest, lay low.