How did you notice them? Standing with feet together and tapping one foot on the floor, it's easy for the water displaced by your tapping foot to hit your other foot and fool you into thinking that you're feeling vibrations in the concrete, as though from a hollow spot. If that's what you're feeling, nothing to worry about -- and you can confirm that the spots aren't actually hollow by tapping on them with something that won't displace much water or harm the floor. Something like a broom handle, maybe.