Coolant problem

I was changing my oil today and checking all the fluids and low and behold the coolant overflow was bone dry. Since I just bought the car I thought, no big deal, I'll top it off and be good to go. I just happened to have a thing of prestone 50/50 mix with anything coolant that was 3/4 full so I put that in there. ALL OF IT. The SOB sucked it down like a whore sucking down a cock after being stuck at a nunnery.

So I ran down to Autozone to get another thing of coolant and the damn thing took ALL OF ANOTHER CONTAINER. Bought another one and it took maybe 1/10 of that one to stay at the full indicator mark on the tank.

My question is how low would the system had to have been to have to put in that much? Why didn't the coolant light every come on? The car never overheated. The needle on the gauge stayed at 12 oclock except for once. I was running at 60 MPH for about 45 minutes and it shifted to maybe 1 oclock. As soon as I stopped it went back to 12 oclock.

I'm letting the car sit over night to check for leaks. Anything in particular on an 04 GT to check for coolant leaks? I know on my old focus the thermostat housing was almost everyone's problem with coolant leaks. Anything like that on these?

Also is it possible that it's just never been checked and just got used up over time and there not be a leak or head gasket or something like that?