That happens when trying to access the payment page for a basket that doesn't exist. Shouldn't be anything to worry about. It can happen if someone sits in the basket and lets it timeout, which should take hours, or if a spam bot is trying to access the payment page with a fake basket id for example. It's basically just an access check and is why it's marked notice.

CBSubs is aggressive with its logging. Try not to obsesses over the logs too much. The biggest ones to monitor for are actual errors.

Yes, as errors could completely interrupt a process. Warnings, Notices, and Info for example shouldn't be a problem and is generally just to give a record of changes that could help diagnose a problem when an error does happen.