Hi,
I have Instant Book turned off because we have had too many double-booking issues - mostly that Airbnb servers are far too slow in updating with our Elloha management system (Booking.com and others are almost instantaneous whereas Airbnb takes 30 mins or more).
However, periodically Airbnb will just accept a booking request with no prior notice or validation whatsover. It happened to us again last night - 3rd time now.
Airbnb customer services admitted that there must have been a technical glitch that 'temporarily' turned on Instant Book - its not good enough. We don't have recurrent technical glitches in our settings on other platforms.