Here are the details about Spaulding that would pique Holmes' interest, making him suspicious:
* His age and appearance: Spaulding is described as a young man, seemingly around 20, but his hands are described as "hard and strong," suggesting someone older and more physically robust than his appearance implies. This creates an immediate discrepancy.
* His unusual demeanor: While working in the pawn shop, Spaulding often seems distracted and withdrawn, gazing out the window and seemingly uninterested in the work. This suggests he's not fully engaged with his role and has something else on his mind.
* His suspicious behavior: Spaulding exhibits odd behavior for someone supposedly employed at a pawn shop. He shows no interest in the objects brought in, and he frequently leaves the shop for seemingly no reason, even in the middle of business hours. This suggests he has a hidden agenda and is possibly using the shop as a cover.
* His lack of knowledge about his employer: Spaulding seems to know very little about his employer, Jefferson Hope, despite working for him for a long time. This lack of personal information is unusual and raises suspicion about his true relationship with Hope.
* His "connection" to the police: Spaulding claims to be a friend of the police, frequently mentioning their names and hinting at his "insider" status. This seemingly unnecessary boast could be a way to deflect suspicion or, conversely, a way to draw attention to himself.
These details, combined with the fact that the pawn shop is a potential alibi for Hope, make Spaulding a very interesting subject for Holmes's investigation. The story ultimately reveals that Spaulding is a pseudonym for the vengeful John Ferrier, the father of the murdered woman who Hope seeks to avenge. This reveals the true depth of Spaulding's hidden agenda and the intricate nature of the case.