Ford Fiesta losing coolant

Hi,
i bought my daughter a uk ford fiesta Lx 1.25 zetec manual car 1997 (r reg), 88,000 miles on clock,
it keeps losing water, I cannot find any external leaks, I have noticed some rusty coloured dried water marking on the low rightside of the engine bay as you look from the bonnet, but I have changed the top rad hose which connects to the engine in this area which may have caused this, I have also replaced the reservoir cap, still no good, im needing to top up the coolant reservoir every day from min to max. Sometimes quite a bit more, The oil seems fine, the car runs lovely, no overheating, any ideas,
thanks,
hoops.