Coolant Disappearing?

I'm needing some direction here. After being filled to the cool line, my coolant drops in the span of about 400 miles. No sign of leaks anywhere, no smell of coolant anywhere, no sign of it burning in the exhaust, it's like it just vanishes.

Once it reaches this level at the bottom of the reservoir it doesn't go any lower. There is no boiling, bubbling, nothing. Coolant temps are normal and nothing seems effected by it, but I'm trying to figure out where to start here. I don't want to start pulling things apart if someone's seen this before. I'm used to coolant issues being pretty straightforward to diagnose but I'm scratching my head here. Maybe I'm just missing something.

 

You might check to coolant level in the radiator once the overflow tank is empty and your engine is at ambient temps.

 

I'll check that when I get home, good call.

Yeah it's too fast which has me nervous. I took a look around and there's no sign of a leak anywhere, but I'll get in deeper and check that weep. I just changed the oil about a month ago and levels and color were normal, so I doubt it's a head gasket, but I'll run it just in case.

 

Overflow tank can’t get lower than that.

Look for obvious leaks on all the coolant plumbing; hopefully no sweet coolant smell in the cabin. Check for valley plate heat exchanger leak… it’s hard to see directly; tell tale sign will be coolant streaks on the back of the engine to transmission bell housing