I had a funny experience in Waitrose (a local supermarket) a while ago. I guessed that one of the cashiers was Japanese from her appearance and accent, and when she returned my credit card to me I held both hands out, palms up, as if someone had passed me a weapon, and bowed my head slightly. She automatically said something in Japanese in response, which ended in "sensei". It felt very natural at the time, but by the time I got back to the car I started to see the funny side of it.
When I was in Tokyo a few years ago, the stallholders under Shinjuku station were all calling out to potential customers as "sensei"; and when I used to go to a Japanese osteopath I always addressed him the same way. I understand that in the context of Japanese culture it is used as a general expression of respect for someone's experience and seniority.
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".