And just to be clear:
spawn points are
1) the spawn tiles placed in the game set-up AND
2) in each building zone when you open the door to it for the first time right?
2b) each building zone if you open a vault into that building before you've opened a door.
3) any spawn tiles added by necromancer spawns.
@Talljohn - using unadjusted rules, the times you might not place zombies on a spawn zone is if you draw a card for that spawn which is one of:
a) an "extra activation" for all of a given type of Zombie
b) a necromancer or abomination when all of that type are already in play
c) the card is a "Double Spawn" which you apply to the next spawn point.
But you always draw a card for every spawn point.