I'm sorry to hear about your night time visitors. It's possible the water has satuated to soil and so now there isn't anywhere for the water to drain so it appears that the pond isn't losing water.

Here is a copy of the article on finding a leak. I hope it helps.

Finding a Leak
The primary function of any pond is to hold water. Leak checks are a vital part of any start up. For a still pond the obvious question is Does it hold water? If so then everything is set to go. If it doesn't, then it is a simple matter to allow the water level to stabilize and then inspect for damage at the stable water line wherever it may be.
Lowering the pond level an inch or two below the stable water level will allow some additional space for inspection. Careful inspection is in order for all ponds. Inspect liners for deterioration from ultraviolet exposure or physical damage. Tubs and preforms should have no problems due to weather conditions, but may need some adjusting of level or back fill to support the container. Concrete ponds in freezing climates may have been subject to freeze damage or settling if not on a firm foundation.
The key issue for many ponds is finding the exact leaking area. Whenever there is a system that moves water out of the basic container there is a risk of leakage. Often these leaks can be difficult to detect because of the nature of the pond, filter, stream or other feature. Ask: When was the leak first noticed? How often is water added? Is there an automatic fill device? Is there any damage or settling? Is animal damage a possibility? Children playing in the pond, or well-meaning landscaping near the pond may result in leakage as well.
Next, take a walk around the pond to look for any obvious wet areas, filled soil berms or retaining walls that may be subject to settling or movement. Gradually increase the detail of the inspection as obvious possibilities are ruled out. If the details of construction are known, the inspection as obvious possibilities are ruled out. If the details of construction are known, the inspection can focus on finding the leak. For ponds of unknown construction, the inspection may need to include some additional details about the pond to know exactly what to look for. Keep in mind that splash or evaporation should be less than five gallons per day for the average water feature of about 8 x 10 feet. This amount can be scaled for smaller or larger features.
Isolating the systems is the key to finding the leak point. If the leak appears when the pump is running, then turn off the pump to see if the containment leaks at all. Be sure to start with the pond filled to the overflow point. Even very small cracks or holes can leak substantial amounts of water. Allowing the pond to stand will either identify or eliminate the basic containment as the leak point. Small leaks may take a long time to reach a stable level. To speed up the process, it may be helpful to pump out a few inches of water and then re-check for a constant water level. By lowering the level a few inches at a time, a relatively narrow band of pond containment can be isolated for inspection.
If the basic containment is intact, the next step is to isolate any other system components to check for leaks. Piping and filters can be checked by filling the system and shutting off the pump with a check valve, gate valve, or pipe cap to prevent back flow and then watching that part of the system to see if the level drops. This operation can be done at the same time the basic containment is being checked.
If there is a waterfall or stream, that part may best be examined with the system in operation. Installed liners make this process easier since any leakage will probably be at the edges. As needed, a hose or pipe can be placed in longer streams starting at the bottom to isolate portions of the stream to check for leaks. Concrete watercourses without liners underneath may be subject to leakage through cracks or settling of the stream bed.
Once the leak is isolated, the correction is usually quite simple. If there is liner damage, a patch appropriate for the liner type will provide for a permanent repair. Component or piping leaks are usually a simple repair or pond edges are often a simple fill or restructure and replace any decorative stones or plants. Depending on the time of construction and the nature of the site, the first winter may have significant settling of berms and edges. These areas are the first place to look for leaks and often are the source. Edge leaks will generally leave evidence as wet spots along the edge. Dry days are the best for checking pond integrity.
The obvious goal is to build ponds with no leaks from the start. By building a pond with a variety of inspection points around the edges, it can be very simple to check pond edges for adequate liner height. - Fold some liner under the edges to have some additional liner available if settling occurs or the owner desires some small modifications. Also check for liner folds that may allow for channeling of the water. Any folds should perpendicular to the edge of the pond or steam. Folds at an angle may have a section that sags below the water level and produces a leak path. Also be sure than liner folds in the stream overlap downhill like roofing shingles to prevent channeling in the stream bed.
For those involved with concrete construction, placing a rubber liner under the concrete structure will prevent leakage in the event of any cracks in the concrete shell. Even concrete with applied coatings can benefit from being installed over a liner. Any waterfall or stream should have a liner that overlaps the basic water containment.
Try to use the maintenance activities as an opportunity to meet with the owner. Ask if they are satisfied with the performance of the pond and the overall environment. There may be a desire to upgrade the systems, install a more efficient pump, or change the companion landscaping. Also answer any questions the owner may have regarding the pond and routine maintenance functions that could make the pond experience more enjoyable.
Article courtesy of John Menhart, P.E., Pond Sweep Manufacturing, Company

Raccoons may have pulled the hose off the pump so that it drained. You may not have a leak.

This happened to my father three times and my father in law many times.

I assume they do it by accident the first time but learn that it makes it easy to get the fish and they do it on purpose the next time.

Check all your plumbing connections. Never place your pump flat on the bottom so that it can't pump all the water out of the pond.

I'm sorry I don't have an answer, just a suggestion about deeper ponds and swimming pools...
It's always a good idea to put something stable in the water for an animal or bird, even a small child to climb to safety, especially those ponds that have steep sides or lined with a stone overhang.

I hope you solve your problem. Good luck.