nuzuki, good evening.
As a rule of Thumb, after placing the Concrete, floating of the top surface to the desired level / finish ans after the Concrete has "Gone Of" meaning you can stand on it the top surface of the Concrete should [have] been kept WET, meaning the area of new Concrete ideally should be under a few MMs of water.
In hot weather the convention was to place hessian over the new surface and allow a hose pipe to play water on to the hessian and hence the entire top surface of the Concrete, just to keep it wet.
Why? it was the practice back in the day to undertake the above so as to prevent Micro-Cracks forming on the top surface of the Concrete, and to assist in cooling the Concrete down, the Concrete cures by a Chemical reaction that itself creates heat, in a large thick pour the heat build up is immense.
As for what you heard? probably the very top surface grabbing the water because the water within the top few MMs of the Concrete has sufered a loss of water due to evaporation [exacerbated by a breeze and heat] so is screaming out for water to replace the evaporated moisture in the top surface of the Pour.
In general terms, a lack of water in the hours following the pouring of the Concrete may, I stress MAY??? lead to a more than usual dusting of the top surface of the Concrete, apart from that the lack of [dare I say it] the Curing of the Concrete, the slab will be more than fit for purpose, if it has been designed correctly.
Ken.
