My daughter was married a few weeks ago and two of my cousins attended the wedding. After going through all of their gifts she told me my two cousins did not give a gift or an envelope. She wants to send out thank you notes and wants to make sure an envelope at the reception (from the cousins) was not lost or stolen. If they truly did not give a gift, she won't write a thank you note but how can we tell if it were stolen or lost? She doesn't want to put them on the spot by asking.

Ask a relation close to them ? Send them a general thank you for being there.

Send one to them anyway. If they say thank you, then something happened to the gift.

If they say, "What gift?" Just respond with, "Your presence was a gift, thank you."

A rose would be a romantic gift to your wife