Actually, FTA:

>The dog became alert, indicating it sensed marijuana.

>“We figured we had something,” the investigator says.

>But they did not have enough to secure a search warrant.

My bad, you're right. They went in with the excuse of investigating illegal dumping.. That's when they came across the hidden door and stairwell --but that was instigated by the triggered dog. Else they were going to wrap up the investigation.