Quotes from this book
Nagataka's grand son Kiyoo said "I was told my grand father disguised as candy man Sasa-ya from Edo, investigated illegal trading items in Niigata."
Kawamura's family crest was Sasa(bamboo leaf) , the story convinces but it also sound too good to be true.
Nagataka's report was so detailed and the search area was extensive.
I guess he interviewed group of spies (not Oniwaban) and wrote the report.
Possibly this report contains copy of important documents of Nagaoka-han. The report was unnaturally accurate.
Obviously it was not something he heard from Inn's owner.
This report was result of mobilizing enough people(spies) and took enough time.
Unlike the myth, he didn't go to Gofuku-ya(Kimono shop) to disguise right after receiving the order even in the case of urgent.
It was impossible to leave Edo without preparation such as researching informations of the destination.