Renault Laguna losing water

This is likely the radiator overflow tank having perished and has "micro-cracks", with small cracks around the hose entry at the bottom of the tank. Just had mine replaced after the same problem. It is located on the firewall between the engine and the passenger compartment, right hand side of the engine bay. It is a white cube. Park car in darkened place (car park or garage). Use strong torch and shine light from top into the walls of the tank, so the light passes downward through the side walls. If you see little cracks then your tank has perished, and you will have to go to Renault with all your credit cards, and firstborn to buy a new one. It is a pain to get out and change over (so my mechanic says). Likely carking points are at the bottom of the tank where the hose goes into it, and you can't see it (you could try a mirror on a stick).