Check lower radiator hose, looks like this and has 2 plastic T joins that leak.
Changing this hose will make you swear alot too!
This was the cause of my v50 slowly losing all of its coolant, tank empty, gauge normal no low coolant level lights.
Silly design.
As ramrod said, the coolant tanks also fail quite often, I'm on my 3nd one in 7rs of ownership (08 v50), altho I did cheap out and get my current one from the wreckers