Losing Coolant / Water but no other symptoms :|

[bbg404]

Morning all not sure what made me look but in the last 4 years of owning this beaut of a car I've never checked the coolant always just have it serviced once a year nothing mentioned about it being low or anything never noticed a problem anyway last weekend I had 400 mile round trip so thought I'd check the oil and just top it up as I did so I noticed 2 things the oil was on the low marker and so was the coolant

I topped up the oil and after the trip I let it settle and it's still st the same level so if it's burning oil it's very very small amounts

But the coolant was topped up to just below max before I set off I got home and checked the coolant once it had cooled but it had went back down to the low marker exactly where it was before I topped it up!
I've had a quick search on here but I'm not getting any of the other symptoms ... Over heating, heaters not as good, temp gauge is perfect

I read on a different page it might be a faulty tank cap?

Any other ideas? Before I put it in to the garage