Took Cass to the Lucky 38. I like to solo and sneak a lot, and having a companion go nuts the moment an enemy appears doesn't lend itself to my style of careful battle prep. Soooo I tell her to wait here.

Head in for my second foray to Vault 34, having been ill-prepared my first trip. Finally go down and make the final decision for my quest. Go through a door back up the first level, turn around to scan the area and - ARRGGHH!!! I wet my pants. Almost shot her head off. Yup, there's Cass, crouched behind me like she'd been there the whole time.

Stay HOME!!!

Now I've got to make my way out and send her back, and I'm guessing she'll just show up randomly again.

Needs fixing...

Ahh, you should not have told her to "wait here", but rather, you should have told her "it's time to part ways" and then "Stay at the lucky 38" (to do this, you also need to choose the "talk to" option in the companion wheel). In some areas, companions will appear even if you told them to "wait here" earlier. This doesn't happen if you choose the option to part ways.

Also, you can do this anywhere, your companion will return to the Lucky 38 by herself, and wait for you there. So no need for you to travel to the Lucky 38.

Bug with Vault 34 itself. Followers will, by magic, teleport to the inside of that vault. Even if you leave them on the other side of the map locked in a room.