As others have said, you most likely are seeing condensation of summer air humidity on the relatively cool slab floor. Air dew points then range from mid-upper 50s (a very nice "dry" summer day) to low-mid 70s (oppressive humidity). Meanwhile the soil under the slab, particularly with no winter heating, likely is well down in the low 50s. So you get condensation on the tile, and since the tile surface is not porous, as just concrete is, you get a film of condensation, likely a slip hazard. I would guess, also, that there is no sub-slab insulation, which would have prevented the problem in the first place.
One solution would be to add a layer of foam insulation, a poly vapor barrier, and finally T&G plywood, screwed through to the slab. Over that would go flooring of your choice. The big problem here is that bringing up the height of the finished floor could well be above the bottoms of doorways. Moving doors up would be some work.
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