Similar thing happened to us for our Wedding actually.
One of DH's friends gave us a wedding card, but it was one of those cards with the slots in for money or a voucher to slot into but there was no money or voucher.
When we were writing our 'thank you's', DH had to ask his mate if there was supposed to be any money/vouchers in the card as we wanted to send him a thank you note, and he said no, it was just a card.
So he must have ran in and picked the card very quickly not realising it was a voucher type card.
It was embarrassing for DH to ask him!!