I got married over the weekend (yay!) and my husband and I just went through all of our wedding gifts. All of our gifts were cash/cheques in envelopes from our guests as we didn't have a registry and it is customary in our culture to give money as wedding gifts instead of physical gifts.
Me and my husband are good friends with another couple that attended our wedding and when we opened up their card, we were shocked to see that there was no money inside of it. I completely understand that there is no obligation for guests to provide gifts, but these are good friends of ours and it seems out of character for them to just give a card with nothing inside.
In fact, we attended this couple's wedding 8 months prior to ours and we gave them a generous monetary gift in the form of a cheque. The couple has since cashed the cheque and sent us a thank you card for the gift so there's no chance that this is some sort of retaliation for us not giving them a gift for their wedding. Further, when mutual friends of ours and the couple in question got married last year, this couple was not able to attend the wedding and the mutual friends told us that the couple still gave a monetary gift for the wedding even though they didn't attend. So you can see why I feel like this empty card is likely to be just an error or oversight.
My question is, do I approach this couple about the fact that there was no money in their card? Or should I just let it go? If I do say something, how do I say it politely?