They use the Jacobsons organ to sense what is around them, thats why they are always flicking their tongue in and out.Question is, how did she know there was water accessible up there? My theory is the water was cooler, it being morning, and she sensed the temperature difference on the outside of the container.
There is also a 150-gallon stock tank with water practically touching this container, but that water is dark, dirty, and funky compared to what's in the big container.
They use the Jacobsons organ to sense what is around them, thats why they are always flicking their tongue in and out.