So I decided to do a lot of exploring before focusing on the main quest. I'm level 18 now and just got to Rivet City.
Quest Pointer leads me to Dr. Li, but when I talk to her, she has nothing to say about dad.
Have I done something wrong, or am I too high level or something?
It may be a bug. have you been to the Jefferson Memorial which is right next to Rivet City? Read the spoiler if you need to know what to do.
Spoiler
She tells you your father went to the Jefferson Memorial to work on Project Purity and asks you not to go, that it's too dangerous. You go anyway and find Holo tape #10 in the rotunda. Listen to that tape and dad tells you where he went off to and why.