Would you try to replicate the issue yourself - fill the bath, have a look at / feel the pipes underneath it esp at the joints, see if you get drops of water... is there a shower over the bath? Would the seals around the bath be leaving water through?
Failing that it's onto the sink / toilet in the bathroom to see if they are giving the issue.
Ideally, it's a plumber you should be calling for such a job... but between call out charges & length of time to find the issue, the cost goes up... anything you can do yourself to find the issue, the better...