The thermostatic cartridge seal might of perished and started leaking.
http://www.aqualisa.co.uk/upload/PARTS%20STORE/EXPLODED/Exp17.jpg
(see part 5)
Or if there is lots of water, the housing might of cracked allowing water out (part 4)
The easiest way is to open it up to find out
                
 Ric
                
 August 2008