The good guys are having a hard time finding a lead for the current investigation. They find a possible culprit or a vital link in the investigation. The team gets an address and find the door slightly open. Too bad whoever they find behind that door, be it a suspect, relative or complete stranger, can't help them close the case. See also Your Door Was Open, Open Says Me; contrast Locked Door. This is a Death Trope so don't be surprised by spoilers behind the ominously open folders. Examples of Ominously Open Door include: