The snarks do apear, but as you probebly know they die after a few seconds. use a
monster maker to spawn them when the player gets close. Use
trigger_once to trigger the monstermaker. The trigger_once is a brush based entity, it defines an area that when the player passes through it, it will activate an entity or a whole procces.
Create a brush textured with the AAAtrigger texture over the area you want the trigger to apply to. Choose the block and press Ctrl+T, write the name of the monstermaker(give it a name first) in the 'target' value. The trigger_once entity will be invisible and passible ingame. Trigger_once entities can only be activated once, Trigger_multiple can be triggered as many times as you like.
The snarks will be spawned when the player will pass through the trigger_once.