Yes, exactly. I tried this in another instance here, when translating "Falls" and it wasn't accepted. "In case what" just sounds weird.
It makes sense in the context of "Pack an extra bag, just in case." "In case what?" I have no idea, however, if the German translation is meant to be in the same context.
Your reply is like a contraction, tho. It may be common to spoken language, but not the written word, which is really what we are all learning here. It woulda coulda shoulda still been "In case OF what?"
In case of emergency! But I see your point. :)
I put "If what?" and Duo accepted. I wasn't entirely comfortable with that answer either, but I chose it as the lesser of two odd sounding translations.