Thats not a good thing mate the metal one I always have is filled to just below the cap level. the plastic one I have about half full but it's just an expansion.
Now you either have a leak or could be worse headgasket failure :(
Have you seen any visible signs of coolant. Is it struggling to start in the morning at all ?
the bit's in the water tank could be from head gasket sealer, meaning the person who sold it knew about it, or could just need a clean through with fresh stuff but with what you have said so far I'd say HG mate :(