We did a maintenence job on a flat roof for an apartment building recently. Sealed all pipes/skylights/hvac units etc sine the roof is still in good condition and did not show signs of cracks or bubbling etc anywhere. But when it rains, a few of these units reported leaks and after checking the roof numerous times, we can't see any place where water can come through. We walked the whole roof and checked everything and see no signs of water going through. The only thing I can think of is an issue with the drains over the units below. Everything is coated around the drains, so could this be a plumbing issue?