Why not "was hat im Krankenhaus passiert"?
This is a dependent clause and verb goes at the end. This distinction haben/sein is not clear to me.
Motion verbs use sein, while non-motion verbs use haben. That's about 90% accurate and extremely simple to remember.
Ich bin gegangen. (bin + motion verb (go))
Ich habe erzählt. (habe + non-motion verb)
Yes, but I did not think "passieren" is a "motion" verb !! Is this an exception ?
I agree with legX, and actually in my dictionary it lists passieren as using haben, not sein. Please help if you can