I personally lean towards the Reddit theory we're not allowed to discuss in detail. But the "chance encounter" scenario you describe, I think, is a good close second.
People still go "cruising" for sex outside without using apps or online forums, and the middle of the night would be ideal to look for a quick, discreet hookup. Danelle Hallan, in
her coverage of this case, said this area has a lot of college students, so maybe someone stalked the area at night to see what student they could find. She also added (and, for the life of me, I can't find the timestamp) that this complex had a lot of empty apartments, which means fewer witnesses and suggests that an empty apartment could've been selected as a hookup spot.
If this was a "hookup gone wrong," I doubt the actual encounter happened at or near the complex though, as nothing has been found there. I think it's more likely the person lured Caleb into a car. The person could've given an excuse like, "It's too risky here. I saw cops nearby. But I know a better spot" etc.