Sorry, Need your help again.

I am writing out my Thank you notes from my wedding and I have a slight problem. My cousin brought an envelope with cash in it to our wedding but accidently left it on his table. We never got the envelope and we are assuming that one of the waite staff walked off with it. We told my cousin not to worry about resending a gift (his parents were extremely generous) but I am not sure how to word the card. We having been thanking people for their generous gifts but he knows we didn't get his envelope and we don't want to harp on the fact that someone walked off with it.
What do you think?