Is it possible this was all a computer simulation? After Sebastian injected himself with the invisibility serum he was in a LOT of pain and could have passed out. Knowing what kind of a person he is and to see what WOULD happen if he was able to explore his invisibility, the others put him in a virtual reality simulation helmet and watched on a monitor to see what he would do. After finally making him visible they would wake him up.

Unfortunately there was no end credits scene showing this, but I think it's very possible.