Have just discovered we may have the same problem. Last year after what I thought was a leak from pipes under sink and row of cream porcelain tiles turning black plumber was out twice, fixed sink and from then on I have noticed a terrible dampness and mildew in sink press. Plumber out yesterday and said this could be from leak under floor and yes it could be an extremely expensive job depending on where leak is originating.
Tiles or wooden flooors may have to come up and in our case, I dont think our insurance will cover the cost of all the work plus new tiling. They told me they will only pay to replace the line of damaged tiles right beside skirting under sink.