Hi,
Could be the waste under the bath leaking. Slide a baking tray underneath it and check it after emptying the bath. Also check the waste pipe along it's length. Could be a bad joint. Again fill the bath pull the plug and run your hand along the bottom of the pipe
Could be water getting down the edges of the bath from the shower. Leave the panel off run the shower and shine a touch under the bath if theres a leak you should be able to spot it.
Hope this helps.
J Brown
March 2007