That's a great idea on the floating container, It should have close to the same evaporation point as the pond, if far different, start looking for a leaking point, another is let the pond be and see if it loses down to a point and then slows down drastically. If that is the case that is the area that needs to be looked at closely as this is a indicator of where the leak may be. Particular places to watch for slow leaks, along waterfalls and streams, where piping is made up, if you have a skimmer check where the water enters the skimmer.
Also if you are like we are here, NO RAIN in weeks, look for plants that are thriving or green grass where everything else looks dry and crisp. With it losing a pretty much given amount a day I would say a good bit is evaporation and if there is a leak probably it's going to be a drip leak or splash out. Along the stream or water fall move a rock or two and look and see if the ground is moist underneath. I recently found a leak in a pipe joint that was made over a year ago, It's probably been leaking since installed but just got bad enough that I started noticing moisture where it should have been bone dry.
Be Aware, that if this is a fairly new build some settling can occur, if this happens a wrinkle along the edge can be the culprit and in most cases finding it is the hard part, fixing is usually the easy part in those cases, a simple rock or a handful or two of dirt pushed under the liner where the leak is found will fix it quickly.
