What are subtle signs a couple is about to split up/get divorced?

One is really disengaged. We were on vacation once with another couple and the guy was his normal self but his gf kept herself kind of separated from the rest of us. She didn’t want to hang out or participate in anything with the rest of us (she had not been this way before) and I knew something was up. About a month later, she left and starting living with another guy.