Yes, you are right; it should be like this:

"There was a car shed on the ground floor of the building. When Ronnie lifted the shutter of the car shed, he looked around to see if anyone was watching him."

"Yes, there was a half-finished coffin there, which he could guess was Ronnie had finished half and came to do the rest of the work."

Now I've changed it. Thank you for pointing it out