To older people such as myself (who were using the words before you younger people were), “no problem” means “the problem you might expect this situation to have caused is in fact not there”.

It’s for when someone’s gone beyond what they owed you.

A barista owes you that coffee; it’s their job. You are literally, as a paying customer, welcome to that coffee.

But someone who has asked a fellow patron to watch their laptop while they go to the bathroom, has received a favor beyond what the roles make expected. This could be a problem, hence the saying of “No problem” to nullify the implied question “Is there a problem?”

It’s kind of like the way someone might report “No injuries” after a crash (which could conceivably produce injuries).

It’s the spoken second half of this unspoken exchange:

“Problem?”

“No problem”

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