The warning sign appears in the booking page when one or all of the below conditions are met:
1. Local Guest Booking: Guest creates a booking in a city he/she lives in.
2. Last Minute Booking: Guest creates a booking which arrival is in the next two days.
3. Weekend Booking: Guest creates a booking on Friday or Saturday for just one or two night(s).
When you click on the warning sign next to the Booking Details title, a pop up window appears.
You can remove the warning sign by clicking on Hide button or re-set it by clicking on Show button and the sign will disappear or reappear accordingly.
Unless you hide the booking warning manually, they will be available in Tasks-> Host -> Arrivals.