Ok, so in an effort to get power armor and finish the Cass quest at the same time, I go to the Silver Rush and leave Cass and Rex outside. I wanted to clear out the Silver Rush before turning in the evidence and making an oath not to harm them. So, I clean out the Silver Rush (after 4 tries), walk out of the building and Rex is the only companion there. Cass has vanished. I went to the Mojave Outpost and she isn't there. Anybody know why or where she runs off to?

Most of the time when a companion isn't with you they're in the zone you just came from, go back and look for them there, try using the wait feature to see if they'll make their way back to you on their own.