Local police looking for a body, decided to drain a pond. I assume you don’t just put in a vacuum hose and flood neighboring property. Tanker trucks?
Ponds have dams…you just backhoe a trough and drain it.
Well, ponds around here, anyway…plenty of natually occurring ponds in other locales.
You got to ask yourself how the pond formed in the first place. Natural ponds almost always have an inflow and outflow. Artificial ponds usually include either a drain, or nearby access to a storm drain system.
Still, good question. I’m curious what the standard procedure for this is.
Where I live, if they have to de-water a retention basin fairly quickly, they use pumps and pump the water to an appropriate area nearby. This may be another basin, a stream, vacant land, etc (with the owner’s permission, of course).
My homeowner’s association has a pond in the green belt area. This only exists as a pond because of some boards and rocks at one end of it, before it continues on into a natural creek. When the HOA board decided that we had too many ducks, one of their measures was to take out one set of boards and lower the water by about 6 inches (this took two guys with hammers about fifteen minutes). The excess water ran down into the creek, which I doubt was noticed by anyone downstream.
Obviously, this solution doesn’t work for every pond, but some ponds are trivially easy to drain.
You swim to the bottom and pull the giant plug. Does no one watch cartoons anymore?
The actual answer depends on many things including capacity of the pond and other variables mentioned before.
There are over 1,000 ‘lakes’ and ponds in my 35 square mile city.
For the most part, my bet is that they’d pump it into the storm sewer. One not connected to the pond in question. 
Pumps are a good solution. Hopefully there aren’t fish in there. It could be a unfortunate effect on the environment to drain a natural lake with fish, frogs, snakes and other critters.
Here’s a random design for an outflow control structure. I just googled for a picture that matches … Its not a picture of the entire pond, it is just the Pit at the overflow which is covered so that people don’t get sucked in, and so they can collect the garbage out , its like an S bend to stop stuff going down it !
See that vertical pipe ? its for adjusting the height. Just chop a bit off to drop the height a little bit. Or remove the vertical pipe to totally empty the pond…
My parents just had to remove a broken dock from one of their ponds. When it was yanked up it pulled up a giant plug and the water was released. Now they’ve got a huge mud pit where that pond used to be.
For a pond to hold water their needs to be a good sealing layer of clay. That is in most soil conditions.
When the post drien in it must have passed through the clay or pulled the clay up when the post was removed.
A local commercial fisherman attempted to build some fish ponds on his property a few years ago but went a little to deep and broke through the clay. I would have thought those ponds would have sealed up by now but they havn’t and its been ten years or so.