Take a payment without attaching a client

On the Collect tab, the client field is optional. The screen says it directly: leave this blank and it is filed under Walk-in.

Use that for retail sales, someone passing through, or anyone you have no reason to add to your book. The money still lands in Your money, it just is not tied to a person.

If it turns out to be someone you want to keep, add them as a client afterwards and book them properly next time.