Board Game Arena

Using exploration technology a Doomsday Machine is spotted but not revealed. Later another player, or same player for that matter, enters the hex where the Doomsday Machine was located previously. However the DM is never there. Is there some rule we're missing, such as the DM is only there the first time the hex is explored, or it only 'hangs' around for X turns after it's discovered, or some other rule?

If DOOMSDAY machine are not ENABLE, token are used.
So when such a token is revealed (not spotted) it is discarded.

So DOOMSDAY are NOT enabled in your game.