Conan is in detective mode, intent on keeping a clear mind and letting the facts speak for themselves (he also knows where the basement door is). He abandons the trolley at the top of the stairs, jumping down them two at a time. The trail doesn't give him any trouble until he reaches the point where it weaves back on itself, becoming an indecipherable mess.

This gives the small detective pause. He crouches down, with the expression that means that he is not going anywhere until he's solved this puzzle.