Hey there!
So Xavier always strikes on the third day, and you've got three tries to get him. Make sure you visit Petra every evening (she should say she wants to talk with you). Have her watch the cameras to eliminate a possibility, and then on the third day have her guard somewhere. If he's going there, she'll catch him.
Each time you fail to catch him, Petra eliminates one of the options herself. So if you're doing this 'watch twice, then guard' technique, the second attempt should only have 2 options, and the last one should allow you to narrow it to a single one.
We also did install a thing in the game which means once you have caught him, on future play throughs, you can opt to skip over the hunt. It's one of the mini games people have some trouble with. (The other skip is available for Selene's content, but that's just because it's long)