What signs or tells does he get on that last statement?

Heightened emotional response in the face.  The level of eye contact and position of hands is of a piece with the story he’s been spinning thus far.

“And the boy? What did he find?”

“Didn’t ask him.”

“I should speak with him. Around what parts did he come to you? Were you near any identifiable landmarks?”

The tobacconist releases his hold on the dog and stands erect.  He looks steadily at The Nameless One. “Won't get anything from that lad.  Think it through yourself.  A boy that stumbled on a murder and got scared out of his wits… would he linger ‘round the place after he’d turned the body and won a purse or trinket?  For what, a few extra coppers from a body wagon?  Rather he’d have run off and not looked back, I should think.”

Again, he's showing some tendency to cover for others, or at least keep them at arm’s length. 

But maybe he is compassionate after all. 

“Your point is fair.” 

“But this isn’t a good sign. We must go at once. Lead me to the place you found me.”

“What?  Now?  Can’t leave the shop.”

“You must certainly do so. I cannot test you for its mark without the crux.”

He softens his mien: gentle, but still brooking no dissent. “Look,” he says.

He expands his chest minutely. Over the course of speaking this next part, he straightens his posture, making himself appear as tall as possible. He has maybe six inches of height on the man already.

His tone remains soft.

“You have no patrons here, now. When we leave, you may either lock the door or leave your hound to guard the wares, whichever you deem best. We will go to the place directly. If the crux is there, I will test you forthwith. If it is not, then you may return here immediately.”

“Should a customer come here in your absence, they will either wait for you, return tomorrow, or make a purchase somewhere else. This is no tragedy.”

“I have rooms at The Heron in Clerk’s Ward. If need be, I will retrieve my equipment from there and return to your shop before nightfall. We will do what we can to secure this place, and I think we will succeed. This is the path that gives you and your hound the best chances of seeing tomorrow, and you must take it boldly.”

Eye contact will be maintained until either compliance has been achieved or until the tobacconist has raised additional objections.

He looks back at the dog.

“Oof.  A hell of a day this has been, too.  Don’t see why the thing should wish harm on me of all people.”

“We’re near a mile from the place.  Best be quick about it.”

“Understood.”

He will cease any further interrogation while the tobacconist is leading him. He doesn’t want to risk accidentally revealing his own ignorance about some important aspect of the case.

Once they get moving he will make a few overtures of small talk, calling upon his scraps of memory as a well-traveled man and occasional merchant. The man was chatty earlier in the shop, and it would be preferable to put him back at ease, if he can.

The tobacconist moves to the shop's window and leans out to forcibly shut it.  He pats his hound goodbye and leads The Nameless One out of the shop.

They pass through a portion of Flint Court that The Nameless One has yet to trod.  The streets are busier, and there is a decidedly higher density of children about, either lounging or barking out sales of flowers, grapes, and phosphor matches.  There are fewer tieflings and giths here than there were near Clapper and Sighs.

A harmonium patrol approaches from the opposite way, fitted in their characteristic spiked red plate.  It's a trio, and the creature that leads them is a rilmani, identifiable by the metallic skin showing between plates on the upper arms.  They look continuously about themselves, but do not stop.

They enter an alley where clotheslines span the gap between second-and-third-story windows of tall wooden tenement structures. The soundness of the architecture is dubious, and the gables are rounded and uneven.  A majority of the signs and advertisements are pictorial, though they pass one saddle-shaped sign that reads simply “GOOD BEDS”.

They come upon busier streets, and then after a few hundred yards, turn once more outside the flow of foot traffic.  They cross under a masonry arch beside a brick facade with broad, iron-barred windows.  As they emerge on the other side into the compound, one member of a gang of street children nearby suddenly turns and runs off, their loose shoes flapping against the cobblestone path.

The Nameless One notices the iron water pump as they pass.  They are moving now upon narrow lanes between apartments.  They come to a T-intersection and turn right, then follow a narrow lane that bends first left, then right.

They reach the place with a jarring suddenness.  The sounds of children playing are still audible in the distance.

The space extends backwards thirty feet between two walls of brick, with two ground level doors standing flush on either side.  At the end is a shear brick wall splattered with an ominous mix of red and black detritus.

A cloud of flies has discovered the remains and is presently feasting on piles of unidentifiable gore in the corner where the street meets the wall.  Even from this distance it appears oddly textured, as if blood were mixed with some sort of mortar-like adhesive or fixative.  It forms small folds.

The wall stains above have a composite pattern about them that calls to mind either multiple violent blows or explosions, rather than the spilling of a liquid or the emptying of a bucket of paint.

The telltale iron smell of slaughter is there in abundance, but after a moment an undercurrent of something else hits his nostrils.

ENEMY.  DEATH.  FLEE.

His breath catches and he feels his stomach retch convulsively. He tastes bile, but nothing further comes up from his innards.

He coughs. “Yes. That is the enemy.”

His eyes quickly scan the ground from his feet to the far wall.  Are there any residues of the carnage spread elsewhere in the alley?