Yep, I've found that too. Popped the hood when offroad a while back to remove a branch that had managed to get caught & push itself up above the axle, & noticed coolant on top of the front diff. Thought at first I'd torn a hose with the branch, but then realised it had been dripping out of the reservoir overflow when I'd been going down some very steep grades.

 

Some evaporation from of the overflow reservoir is normal. Typically, my JK requires about 1 qt per year to keep it filled to the max line when the system is at operating temperature.

 

Yes it's normal. The system is not sealed and you'll get evaporation due to water in the system being heated each time you drive. You have likely only put fluid in twice in over 50,000 miles because every time you take your Jeep in for maintenance/oil changes/tire rotations and other work, they should be checking and filling your fluids for you.

 

Alot depends on your longitude,latitude,heat index and barometric pressure combined gravitational and directional forces applied theirin and toofor the equinoxil hypothesis of astronomical alinement.
In other words, if its low, add coolent, check for leaks, if none, evaporation, tadahh.
You could always have the system pressure tested.

 

Update: So I was checking out the underside of the Jeep and I found a leak. Looks like a seal by the oil filer was leaking coolant. Took it to the stealer and they said the water pump is bad and are fixing it under warranty. Thank God I have the lifetime warranty.