Shiro will be able to detect that these fires are magical in nature, rather than flint-and-fuel. There are no patterns between each hit. Each establishment/item is owned by different people with no known connections other than being in the same time/same place.

There are no scents, necessarily, of spell components! And Shiro will be able to suss out a scent . . . it's a little hard, given there are so many people, but one scent will be able to be picked out. It is human! And Shiro will be able to track it around, losing it occasionally, but it may ultimately bear fruit in combination with other efforts!