Rsvp's

Many people make a numbered guest list and write the corresponding numbers on the back of the RSVP card in case this happens. Once your RSVP deadline passes and you start reaching out to people, you’ll have someone tell you they sent theirs back and you must not have gotten it.

I had about 3 like this so far and thankfully, with process of elimination, I was able to figure it out somehow!
I have 1 more without name/return address who checked off that they're attending. I'm going to put it to the side until I get all rsvps back and determine who it is.
I really am upset that I went against my gut and did not number the RSVP cards smh. Oh well. Lesson learned.

Unfortunately this happens more than you think. As PPs have suggested, wait until your RSVP deadline passes, and begin reaching out to those you haven't received a response from. That should help narrow it down

We used a black light pen to number our RSVPs in case this happened, but we haven't had anyone not fill out their names yet.