This is the most common reason for levels dropping, although it may not seem possible as levels can drop 1 inch per week. Depending on the time of year and where your pond is in the garden, if it is exposed to long amounts of direct sunlight it is possible for significant amounts of water to evaporate.
The only way to avoid evaporation in your pond is to make sure you pond has shade. However, unless you had the hindsight to locate your pond in the shade when building it, the chances are that you have little to no shade.
If you decide to top up your pond with the hosepipe, it is better to do this little and often. This is to minimise the amount of chlorine entering the water. Rather than replenishing all the water in one go, try adding a small amount every day. When adding the water, use the rose attachment on the hose to create a sprinkle rather than a poor. Doing it this way, some of the chlorine will be burnt off before entering the water. If you do have to fill up the pond in one go, be sure to use Chlorine Klear to remove all of the chlorine from tap water and add a dose of Pond Klear after topping up, this will help to replenish bacterial levels.