Not a problem? Hell Yes, there was a problem, actually two problems. I needed something I did not have. That is why I was in the store. I was not sightseeing. Then I could not find what I was looking for. I asked for help. The salesperson provided that help, solving both problems. After thanking them for their assistance they advised me there was not a problem, at least as far as they were concerned.
Would they have still helped me if they thought it was a problem? Was my situation so trivial that helping me barely qualified as a good use of their time? Were they boasting of their mastery of the situation? Was it a subtle put-down?
What ever happen to, "You are welcome"? Or, "I'm sorry you could not find what you were looking for." Or, "Thanks for shopping with us." Or, "Glad to help."
"Not a problem" or "No problem" is becoming an all purpose response and closing remark. It is of recent origin. I have had some choice moments recently thinking of more inappropriate usages. Add your own.
"Doctor, thank you for saving my life."I don't know where it came from, but I would just as soon it return. I am having a problem with it.
"Wow Honey, you were great"
"Waiter, there is a fly in my soup."
"Gee Dad, Thanks for the bike."