And areas and buildings that survivors would find useful and frequent (i.e. pharmacies, libraries with handy how-to books, cleared and arable land, proximity to bodies of water used to purify and drink, etc.), they would police up the bodies and mass-grave them (or somewhere else out of the way), but wouldn't be likely to go building-by-building and house-by-house all over the city to tidy the place up. Just a large enough place to tend to their needs, especially if food was scarce...they wouldn't expend the calories to gather more bodies than was necessary to clean their required space.
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