Losing coolant- faulty cap?

Last week i had no coolant in the bottle, it always has sat really low (well below the min level and has been fine for a couple of years like that) so topped it up, and then this morning again here was no coolant.
Have checked to see if it was the rad but doesn’t appear to be and cant see if its leaking anywhere else. Noticed when i took cap off last week that there was no hiss so indicating lack of pressure, is it unheard of that the cap maybe faulty on the S2?
Cheers in advance
Tanman

Not funny really but what was that discussion we had about both being on the original radiator? Hope it’s a simple fix, on the bright said at least theres Red Post now!