Vidrik's calls and movement produce some shuffles, and, after he has moved on, some puzzled zombies who follow his noise... but no signs of living people. At his knocks on the third building, he's met with a bunch of squeaking and fluttering noises, followed by a mass of bats flying out the window. He freezes and waits until they lose interest.

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1 lower on the stealth check and you'd be rolling initiative against a bat swarm.