Ford Focus water leak

Need advice in finding a water leak, after 80 to 100 miles i have to top up regular. The amount of water seem's to be the same from just above max to between one to two inches below min. Theres's no change in oil color and very small skim of oil just on top of the water in the expansion tank and no change in water color. The expansion tank has been cleaned but their's still a bit of scum in it, the skim of oil on the water in hard to spot , i can only see it reflecting in the light. There's no sign of a water leak underneath motor, the termostate housing and termostat have been replaced and their's no leak. Thank you.