Well, here's my opinion/theory. He/she has been seeing the disappearance of other fish in the tank and the disappearance of the lead discus so he's afraid the next time it will be him/her so he/she is hiding.
What I've noticed in my tanks too is that the other fish take on the personality of the lead discus, if there is one skittish fish they all become skittish, if there is one outgoing devil may care fish then they all become outgoing to match top discus (in pecking order of course). Since you removed this top discus they are falling back on other behavior. In my latest tank of babies they were scared too until I introduced another non-scared fish, now they are all back front and center to great me.
I guess this is my long out drawn out way of saying you upset the dynamics of your tank so this is why he/she is scared. Perhaps a dither or another non scared discus will help him out? Or just wait till he/she regains their confidence?