It should. Being a guard in night duty sucks ass. It's dark, and god forbid if it rains, you can't see anything. And random noised during the night? You aren't exactly in a hurry to investigate it unless someone is screaming like their head is being cut off with a butter knife. So any traces the guards might have found would have been long gone, wasn't when the MC was investigating since he got there quicker.
And yes, telling about the merchant counts as negative. You were told to keep things secret at that point...
But that being said, not investigating it isn't the end of the world as is... well maybe, but not the MC as is...
And I'll check the android version for that issue.
EDIT: Checked the code on both versions, both have marking (normal and WT), and WT version has a mark for the best choice. But changed the icon on the non WT from gaining information to standard Empires masks.
EDIT 2: Played through the part in question, the current WT version does mark it green