Where's the coolant going?

I'm sitting at my favorite bottle shop letting the daily cool down out front. The engine is an S52B32 with a recent cooling system overhaul. Having a 9oz pour of 10.5% barley wine. She was HOT so I've got time to finish this. 

This car has a random coolant drinking habit. I've put 3000 miles on it this summer and it drank no coolant. Today it was 3 quarts down after a 280 mile drive. It did the same thing last year randomly and the year prior. Last year after the issue I pressure tested the cooling system, 25psi for 10 minutes, and it didn't bleed down. 

Where else could the coolant be going? Weird that it waited until cooler weather to drink coolant. Last year it was at least mid summer. 

I'm putting it in storage this weekend and picking up the winter car. Almost like the BMW knows what's about to happen.

Are you checking it COOL? Is your check coolant light coming on or are you randomly checking the level before your drives? Any steam out of the exhaust? Did you use your heater for the first time? Vacuum or pressure in the system when you opened it up?

How recent was any of the cooling system replaced?

It can be a bit of a bear to properly bleed, but 3 qts down has to be leaking or burning. 

Did you use the heater today? Leak downstream from heater control valve that "disappears" for baseline pressure test?

I check it cool roughly monthly and before trips. I only knew it was low when the temp gauge started to move. 

No coolant light. No heater use. No stream. No pressure opening the reservoir. Cooling system replaced 4 years ago. I vacuum bled the system last year when it ran low. 

​​​​​​I like the idea about the heater. That makes sense other than me not using the heater. 

I would rebleed by lifting the nose, running it up to temp with the heater on full blast and filling until bubbles stop coming from the bleed screw. Then check levels the next morning.

Start trouble shooting to see if you can get it hot and watch for active leaks. 3 qts would mean you should have a pretty visible leak unless it's being sucked into the engine (which would put vacuum on the system and steam in the tailpipe at those quantities). Check the heater port on the back of the head for crusty coolant as well as watching the accessory drive for drips. If it was inside the cabin, there would be large amounts of moisture on the passenger floorboard.