2000 Rodeo - V6, 2WD

My Rodeo started leaking transmission fluid a few days ago. It was the weekend so I took it by a repair shop and had them take a look. They said it was the "cerval" cap? Perhaps I misunderstood. No one I speak to about this seems to know what I'm talking about. It's just a little part. They spot welded the part until a weekday when they could order the new part and told me to just keep an eye on it until then. So I keep checking to make sure i'm not leaking fluid. I haven't driven very much just because I'd rather wait until the new part comes in, so I feel comfortable driving it.

I did drive to the store and back and a few other nearby places. Yesterday when I looked under the car the piece that covers this cap (it locks and has a clip that holds it in place) appears to be missing! I'm pretty positive that it was there when I left the shop but it does not appear to be there now. I called the shop this morning and they are ordering the part but I asked if they were now going to have to order this other part. Two real questions. What are these parts called? And is my car drivable without the outer part locked down?

thanks.