Hi,
Apologies for the delay.
It was definitely not a bank charge. My credit card providers (from Singapore and UK, Amex and Mastercard - ANZ) do not charge for using their cards (other than the annual fee which is usually waived :)) unless making foreign currency transactions. Even if they did I would see the fee on my statements.
Airbnb take payment using the card's local currency hence the bank will not classify it as a foreign transaction.
After communicating with an Airbnb community support person it turns out it was indeed Airbnb's currency conversion fee of 3%. I did know about it as it's mentioned on checkout and in my booking detailed receipts but I mistakenly assumed that the amount shown on the payment section of the receipt included this charge as it was already converted to my local currency.
Something to do with the fact that the fee is not VAT deductable so they can't show it in the service fee. To put it simply, the 3% currency conversion fee is charged on the total amount payable after and is not shown. It's a little bit disconcerting that the amount you see on checkout is not the exact amount you will be charged but I have no problem with it now that I know. Maybe some room for improvement for them to make things clearer.
By the way, the Airbnb community support (I was helped by John H) is excellent! He replied promptly, with detail, and was very friendly.
Thanks to all here who replied to this post with advice. Very much appreciated 🙂