Can a bank legally ask where a check came from? I don't think it's any of their business, unless the law says otherwise.
Yes. Your bank has many responsibilities imposed both by ethical standards and by the laws under which they operate. In some instances a bank is essentially obligated to ask about the sources of funds that it is receiving from depositors. While it may not help, the saying "it isn't personal, it's business" rings true here.
There are many cases where a teller would ask about a check a person was depositing. They would hear a story about this being from a sale on eBay, from a work at home plan, or lotto winnings, as examples. All three of these are common scams where part of this money is sent to some other party by the customer. Then the check comes back as a fraud item and the customer is told they have to repay the bank, so these questions have lead to customers saving money, many, many times.
BankingQuestions.com is a free service made possible by the generous support of our advertisers. Advertisers are not responsible for site content. Please help us keep BankingQuestions.com FREE by supporting our advertisers. When you see an ad for a product or service you may have an interest in, click through to learn more.