air in the coolant will cause it to 'use' coolant as the air bubble may eventually pass through and be leaked through to the reservoir and coolant taken to fill the air bubbles spot.
but as stated, if your car is hot, the level in the reservoir will rise, as it cools the level will drop, hot coolant expands, and fills the reservoir, thats why it is there

Thats a better explanation. As said, check your heater core, if its leaking you will smell hot coolant with the climate control on hot and the fan on.
Also, maybe faulty rad cap.
I would go back to a normal temp thermostat, they're not that expensive genuine, and I can't see there being any benefits of a colder one for normal driving.

__________________
Its just the internet dude, its not real.
F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i

I just replaced the reservoir cap couple of weeks ago. Thanks for your suggestions guys. I will check the heater core.
I take the car to the track once a year and a few RAC days as well which is one of the reasons for colder thermostat. I didnt think the level of coolant would be dropping for weeks as the air escaped, but it might be a cause.
Any suggestions on where to get a compression testing kit from?
Thanks again guys,

Adi

supercheap/coventrys/repco/AutoPro

__________________
[CENTER][SIGPIC][/SIGPIC][/CENTER]
[CENTER][COLOR=#ff0000]Built Ej207: Semi-closed RA Cases - Wiseco Forgies - Manley rods - BC 272 cams - GDA Heads - billet f/whl - Vi-pec - WbO2 - 800cc Yellow tops - TD05-20G - 409Hp ath @ 1.75bar on E85[/COLOR][/CENTER]

yeah it wouldn't still be getting out trapped air after weeks. I have a pressure tester if needed. Also check the waterpump which has a small hole that you can often see coolant leaking from near the thermostat area.

there's quite a few other hoses it could be leaking from or even from the coolant y pipe where it bolts to the block, have you checked both the coolant lines to the turbo? the 90 degree elbow to the heat exchanger above the oil filter?

Just cuirous, howmuch coolant have you been using?

__________________
Version 6 STI