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