The underground container is empty when Cassie, Jenny, and the sheriff get there, but Grace had bled extensively inside of it. Additionally, the captives were there for several days, so the stench of their feces and urine would still be present. If Ronald had used bleach or another cleaning fluid, that odor too would be present and suspicious. Any of these factors would lead to Rick being detained and the container treated as a crime scene.
Trooper's hat should have a strap on the back.