In terms of removing monsters from the board if they can no longer be seen by any of the Heroes, I don't remove them today but the idea of doing so once they are too far from the action does sound appealing from a tidiness point of view, there isn't a lot of benefit in having a random Zombie trailing 30 squares behind the Heroes throughout a Quest but I think the problem for me comes with what you do with them after they are removed.
The general principle in HQ is that if Heroes can't see it then it isn't placed on the board, so this idea would be consistent with that and whilst I don't enforce that myself, I do keep it in mind as an EWP to prevent myself from applying some of the underhand tactics, described here, like hiding all the monsters out of sight for a turn so that the Courage spell wears off (surely that is an exploit), or setting up a massive ambush around the exit (a few Quests are set up with such an ambush, why would the designers do that if the intention was that the EWP would set that up themselves anyway).
Although if you do just remove them, there is the risk of a dumb set of Heroes 'losing' the very Monster they have come to assassinate or the Monster armed with the Quest Treasure they they have come to recover, managing to escape. Having now typed that last bit it actually sounds like a good idea, so perhaps removing Monsters that are out of sight and not replacing them is a valid option, might just mean that the Heroes have to replay some 'assassination' quests if the target escapes...Benny Hill theme tune springs to mind
However if you go with the OP's suggestion of removing them and then tracking their movement in the Quest Book, then I don't agree.
ChuckyMcNorris wrote:When playing Zargon, does anyone remove monsters from the board when they are out of site of a hero and track their movement in the quest book? Thoughts on doing this?
(1) It creates what is essentially admin work for the EWP, that builds as the game plays out, increasing the amount of time they spend scribbling notes on a page whilst the Hero players sit there twiddling their thumbs. HQ works for a number of reasons, one of which is that it is fast paced, I could see this resulting in bored players especially as the number of monsters to track grows (less of an issue in PBP games)
(2) If the Hero players cannot see the monsters that why do you need to track them, they take on trust that the EWP places the right amount of monsters in the right place as per the Quest Map which they never see, so why now do we have to track unseen stuff, is the EWP going to be audited at the end of the game?
(3) The Quest Map already shows the EWP player the positions and types of the monsters at the moment when the heroes round a corner or open a door, up to that point logically the monsters could be moving around anywhere, this removes any need for arduous tracking already as part of the standard play.
(4) In addition to the monsters revealed by heroes opening doors and rounding corner the game already has a concept of monsters that wander unseen and untracked around the dungeon, popping up at inconvenient times, the Wandering Monster mechanism. To have unseen and untracked monsters that are revealed and placed on the board only when discovered AND unseen, untracked monsters that are only placed on the board as a result of a search AND now introduce a third set of unseen monsters but these have to be tracked feels inconsistent and redundant
As this would be a vert powerful technique for the EWP, it might star to feel like heroes are moving around the dungeon releasing fair monsters from the captivity of their rooms and then standing aside to let them all escape to fight later.
Imagine the EWP and other players frustration when he spends 30 turns tracking the movement of an ever increasing gang of monsters, only for the heroes to grab the Quest Treasure then exit the dungeon using the Ring of Return!
Personally I think there are better ways of increasing the EWPs tactical options.
by 
= white skull, one "hit"
= black skull, one "hit"
= shield, cancels out one "hit"
