Hi there, @finance134.
Let me provide you the few possible reasons why you're able to see a balance of your supplier even though they don't have an open bill.
First, you'll see a balance if you enter an opening balance while creating the supplier information.
See the sample screenshots below:
Second is if that specific supplier has a credit that is recorded in QuickBooks Online.
You can always open supplier reports and customise them to see the specific details. This way, you can track your account balance, credits, and transactions.
Please feel free to read these handy articles for your reference. This link contains steps on how to pay bills using supplier credits.
Let me know if you have follow-up questions. I'm always here ready to help. Take care always.