I think you are right . ThanksReally sounds like someone spilled something in the pool that is leaving that film. Maybe some type of acrylic floor cleaner or polish.
Maybe the customer had a cleaning person who spilled something in the pool and was afraid to admit it.
Make note of the areas where the film has fallen off. If it does not return then it is a one time event and you just need to get the film peeled off the surface.