I booked a reservation with Days Inn Singapore using my debit card. It didn't as for the MasterCard secure code, nor I see the money deducted from my account. I would like to know what happens with this reservation, since I see the booking is valid at the site. Can the hotel charge me without my 4-digit pin or the Mastercard secure code?
MasterCard SecureCode is a brand name for "3-D Secure" (also marketed as Verified By Visa) and these days it doesn't even ask a password any more which is not surprising because it's one of the worst form of security ever created, here's a paper titled Verified by Visa and MasterCard SecureCode: Or, How Not to Design Authentication.
Anyways it's not a requirement for a merchant to use it. In fact, only the card number is a requirement, everything else (even CCV2, the three digits on the back) just decreases the rate for the merchant because there's less fraud. See the answers to this question on Security SE.