If your degas appeared empty, you were very low, you probably don't even know how low. When cold, you want the coolant at the Max line, because this system is touchy. Since you filled it up, run a couple drive cycles (couple days, maybe) to get air out and coolant through everything. Top off as necessary.

Both directly under the degas tank and the Y connector next to the thermostat are common places for cold leaks that happen overnight. See sig.

I mean, unless you find some obvious leak.

 

Yeah, i think this likely stemmed from being very low on coolant (you'll have to find the leak later) but for now, you need to perform that burping procedure and make sure you've got more coolant ready to top up the degas tank.