I've just experienced this bug for the first time (to my knowledge). I created a male sim and his daughter, then he went to the park and met a woman. They immediately started a date and built a strong romantic relationship. He came home and went to sleep. The next morning I thought I'd invite her over, but she wasn't in the relations panel. I even went into household management and didn't see her anywhere there.
If she's not in the households anymore, you have this issue: http://answers.ea.com/t5/The-Sims-4-Bug-Reports/Common-Disappearing-Sims/td-p/3630991
