I noticed that the flats on Minmus kind of look like lake beds, and some people even call them lake beds. Also, the breaking ground report on Minmus rocks even says that there was once water on Minmus's surface! But how did it happen, and where did it go? Shouldn't a second Kerbin orbit Kerbin?
Theory 1: Minmus Bombardment Era
We know that on Earth, there was a bombardment era very early in our history. It's belived by scientists that comets brought water to our planet. That water evaporated, causing global rain storms for millions of years, creating oceans. What if the same happened with Minmus? Comets brought water to Minmus, and the gases released from them created an atmosphere for Minmus too. But where did it go? In the Real Solar System, Mars lost it's ozone layer, and consequentially, it's atmosphere . This is because it lost it's heat, due to it's size. It's interior stopped churning because heat escaped it. No atmosphere, no water. Minmus almost certainly suffered the same gloomy fate as Mars. Minmus though is much smaller than Mars, so it's geological activity would've ended very early in it's lifetime. Water would've been there, but not for long. It's a very plausible theory, and probably the most accurate out of the three.
Theory 2: Minmus was once part of Kerbin
This one may sound a little outrageous, but hear me out. What if Minmus was once part of Kerbin? While not likely, water could've flown up with Minmus after the celestial body crashed into Kerbin. It would also make sense if that large crater on Kerbin is in fact the remnants of Minmus's formation. This would make more sense, as that crater is fairly recent to Kerbin, a few million years or so. If Minmus was the same age, water existed when the impact happened, and went up with Minmus. We know moons form from planets in real life, too. The Moon was created after a Mars sized planet crashed into Earth. The same most likely happened with the Mun. Minmus is several times smaller then The Mun, and an asteroid the the size of Chixulub crashing into Kerbin would be large enough to create a moon. Ratios in this game are weird.
Theory 3: Aliens
You can't prove that it wasn't.