It's quite possible they know where the water supply is and you just haven't seen them drink. You wouldn't actually notice that the bottle was getting empty so fast because as stated already, they get most water/fluids from their fruit, veggies, and nectar. However, it is also possible that they haven't figured out the water supply, and so I recommend trying what Kferg suggested to be on the safe side.
Just for a future suggestion though, with water bottles it's recommended to have two. I personally prefer water silos,
like these
They can be picked up at Petco or PetSmart as well I believe. I tried these and never went back to bottles (except under a couple of rare occasions). They are super easy to remove, clean, fill, and replace. The bowl is so small that it's less likely to become contaminated like an actual bowl would.