4 Year Old Sometimes Says He Sees A Monster And

4 year old sometimes says he sees a monster and appears genuinely panicked by it. It is short lived and within moments he is fine. It’s always the same monster and comes out of a blind / window of the same room but he’s not always in that room when it happens. He senses it (I think). It is only when he is tired. It doesn’t happen every night or every week just occasionally. Today moments before falling asleep he had to check it wasn’t there and told me that it becomes invisible. He then stared off in to space mid sentence. When I asked him to finish what he was saying to me (after the staring had finished) he couldn’t remember what he had been saying. He is being investigated for Epilepsy as it is not clear if he has it or not. My question although rather odd is, could the monster be related in any way to anything medical.
It’s not the normal monster under the bed situation.