Answer
The answer is as follows. 
Katrielle and the Millionaires' Conspiracy walkthrough
Puzzle Location: Bowlyn Hill (The fire hydrant in the central)
This is the inside of a walk-in freezer.
All liquids freeze in here, but while this man is carrying out his work, there is one area inside the freezer where there is some unfrozen liquid.
Where could it be?
Slide the circle to where you think the liquid is.
There isn't usually any liquid inside the freezer.
It is only there right now.
The liquid is not necessarily visible.
When the man finishes his work and leaves, the liquid inside the freezer will disappear in an instant.
If there happened to be two people inside the freezer, there would be liquid in two locations.
The answer is as follows.