It could be a lot of things in my opinion.
I think they were hip to the general area of where he was working, and with the timing, surmised the disposal site was somewhere nearby. Maybe he participated in past criminal activity at this location (drug deal, etc)...
I'm not sure what electronic footprint info they have if any, but let's say IF they didn't necessarily have anything strong ping-wise, they might have been able to think to look at prior job sites...man, it really can be a lot of things...honestly I'm so worn out from the past 2 days here I can barely post...
In summary, I think the possibilities are endless...something led them there, or at least really close, that's for sure!
Hey, this brings up a thought...someone asked way back if there is any chance WH confessed..it wouldn't take so long for them to find her if he confessed specifically (again, unless she crawled away from the shed maybe). In fact, don't they usually bring the perp out the site with them to show LE exactly where the victim(s) was placed? I've seen that done a lot---Travis Forbes...then there was another young lady recently who was murdered, I can't remember her name right now

, but IIRC and am not dreaming this he disposed of her in a dumpster...maybe I'm imagining this but they brought him to the landfill to search??