Thanks for your response Steve. I've been hosting for close to 2 years now and always have been paid upon CHECK IN, not check out. Hence my concern that something could potentially be wrong with the client's account or something.
Before entering into this community chat, I looked at the Help box, and it confirms that payment usually happens at the beginning, which only made me more concerned. If payment does NOT happen, what's next then?
Sorry about my blunder. I should, of course have said checkin and not checkout.
Have you recently refunded someone? If so could it be that Airbnb is witholding this payment as payment of the refund? If not, then you should contact Airbnb support.
You could check to see if it's listed in your "Future payouts" section.