Yes absolutely, feel free to sense John's whole deal in whatever way translates to your canon! John can perceive an aura of shadow around people or places that are inhabited by an Outer God, so he'd sense an ominous weight and darkness upon Maruki. John represses most of his Hastur-memories, so he'd only know bits and pieces of lore.

tbh snacks are a good start, John's only been able to experience human things like "eating food" since arriving in the jamjar and he is soooo excited about it.