Random diaper smell in Chicago

[Brie Schmidt]

I got a 3 flat in Chicago, when you enter it sometimes smells like diapers.  The tenants say it is on and off for about 6 months.  It only smells in the entrance between the garden and first floor, once you get to the third floor hallway you can't smell it.  The tenants say it does not smell in their units.  The garden tenants do have a toddler but I smelled their unit and it smelled in the foyer but not their unit.  They also keep their garbage at the back door and don't leave it in the front hallway or anything.  There is nothing from the exterior like a vent or area an animal could get in.  Bathrooms and kitchens are nowhere near the entrance.  

Tagging some of my rehabbers to see if you have ever encountered this kind of thing

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