Trapped infected question

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So i encounter walkers that are trapped in buildings a lot, and i still have some questions about it. Lets say someone else could trap this walker in a house, which could be a sign of a player nearby, but what if there was no walkers when i looked at the city from far away (300m+) and walkers just spawned because of me when i entered the city? Does it mean that a walker spawned inside a building by itself due to a glitch? Or does it work the way that even if walkers despawn, they will spawn b...