Gotta need to be Stealthy on this one Detective and Thankfully you only got one secret to find so here it is.
as you investigate the second floor , you will find this room










Look on the side of the small Fireplace and Press the Button , This will open a Secret bookcase and in it you will Find Enough Heat to get rid of goons if things go south.

while searching for clues you will find this bloody handprint










just on the right of it look below the staircase to find tranquilizer gun ammos