Funnily enough I saw an ad for water detecting bed protectors for incontinent people at the hospital just yesterday. They seem to run a material like aluminium foil under the sheet in a pattern. Maybe worth looing into as these seem to fit the bill for what you need. The ones I saw fit to a system to alert a call centre via the phone if the person wets the bed. I didn't see the price though as no need for one (not yet anyway!).