I'm 99.999% sure you misheard something here, most likely something ending in "-mase". It would be highly irregular to call customers "sensei" unless a) you know them, and b) you know they are normally addressed as "sensei", from being a teacher, doctor, lawyer, politician, or writer. Customers in Japan are addressed as "okyakusama", not "sensei". In nine years of living in Japan, in city and country, I've never seen a vendor or salesclerk call a customer "sensei".
Hmm. Maybe I did mis-hear in Shinjuku - I was quite tired and jetlagged at the time.
In the case of the lady in the supermarket, though, I was pretty sure what I heard - perhaps she had had experience in the martial arts?