loss of water in the c.h.circuit?

I have had a baxi bahama in my bungalow now for around 4 years and have a fault with the central heating side of it .i keep loosing water and am forever topping it up if i forget to check and water gets to low then the boiler cuts out and i have to top up with water and reset we have checked all likley areas where we could be losing water but can not find any. the snag is we had the pipes under the floor when we had bungalow built but surely their would be some sign of dampness any body out there got any suggestionsdonald.edwards2 at ntlworld.com