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1) Concrete isn't water proof and is very porous. It absorbs moisture and will eventually shed it at the point of least resistance - typically the bottom of the slab.
2) Is the crack in the concrete or on drywall? Either way, a building - especially one that's 20 years old - will slowly settle and cracks can appear. Normally it's not an issue.
3) See #1. Also, if the exhaust run has a leak or simply vents into the ceiling, that will introduce moisture into the interior of the unit.
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