Your post has me thinking back a little. Ever since Season 4 started and we got past the 1st few days after the big fight ending Season 3, I've wondered about the population of the zombies. A couple of things pop into my head.
1) I think that there are a lot fewer of the normals after the Battle of the Colony. So there are fewer zombies to encounter in general.
2) General TOWTM has taken Team Zombie into hiding. He has established a base of operations outside of the observation areas of Team Human. I think Team Z has been doing recon and surveillance on Team Human for quite a while - even before the Battle of the Colony. This realization is sort of sparked in Michael when he mentions (in Ch36) that they seem to be attacked when they gather in large numbers.
So, it doesn't seem to be too much of a stretch that the zombies have been actively avoiding detection by the humans. They have plenty of room in the greater LA area without going underground or something similar.
But the idea of them having "safe houses" or observation posts near the Colony or Dunbar is a very interesting thought. And it all plays into Michael's statement of how foolish and optimistic they were before the events of ch39. Team Human has been seriously underestimating their opposition, methinks....
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