Re: Wet bathroom floor
Your builder's explanation * might * be the cause .... though I would think it would also show up elsewhere rather than be localized to that one spot.
By chance if any wall around the toilet is an exterior wall I would more inclined to suspect the bottom plate of the wall framing where it sits on the concrete isn't sealed with a sill gasket or caulk ..... or very well.
In which case removing the baseboard around this area and applying a caulk or acoustical sealant caulk along the bottom of the wall where it meets the floor would help preventing cold drafts entering and cooling the floor.
Just a thought.
"" an ounce of perception -- a pound of obscure "