If you run out the west door during the Jin'rokh encounter then the boss will despawn. This happened to me twice while I was running LFR on an alt. I discovered it because I was targetted with focused lightning early in the encounter and ran to the far west side of the room, through the doors to the chamber, which stay open during the first ~30 seconds of the fight. The despawn caused a lot of confusion for people in the raid and nobody noticed that my running out was causing the despawn. I would guess that if you wanted to be especially sure that you aren't seen you could throw down a demonic circle or monk transendence back right after you despawn the boss.