How feasible is it to check out of a HI property if you've already physically left the place?

I've got two consecutive nights booked for later next week at a HI. I've booked an paid Advanced Purchase Rates and will need to use the first night. However, I've discovered (at understandably short notice) that I will actually be attending a funeral requiring a bit of travel to attend for the second day. I could travel all the way back to the hotel that evening, but I'd rather not, considering the event is near to my home and the hotel isn't.

I've signed up to the Stay 2 nights Get 1 night promotion and have a reasonbly cheap rate for the second night (the first night is expensive though) which is why I made the second night booking in the first place.

Rather than cancel the booking (non-refundable pre-paid anyway) I'm wondering if I can simply telephone the hotel front desk in the morning and tell them I forgot to check out?

Do you think this would work if I've already pre-paid? Anyone tried?