I just realized that our degas bottle (coolant reservoir) doesn't have a sensor - crazy!
I flushed the coolant last weekend and I took the degas bottle off and cleaned it as best as I could and didn't even think about how there was no coolant level sensor to disconnect.
There's still a bunch of crud in my degas bottle and hoses from my small air leak that Ford couldn't find (it was a hose connection). I didn't think to buy a replacement reservoir before I started draining the coolant.
From other users' experiences (mine included), the coolant generally runs on the low side of the tank. Typically, when hot, my level is just above the 'low' line, yet I never have any issues. My best suggestion would be to mark on the tank with a sharpie where the coolant sits when hot and when cold. If it drops below those points, you know you've got a leak.