Scenic – battery is discharging – which cable is it?

Hello, have a Scenic 1,5cdi BJ2005, something sucks the battery empty when parked. I checked all fuses under the dashboard, found no consumer. The current flows through one of the red cables at the plus pole of the battery, with changing currents from 0.7 to 5A. The battery is empty in one night. I put it to my mechanic, who “has it not so” with electrics and has no idea where this cable goes and what it is. The cable disappears below r this thick air hose right at the front of the picture. please ask for info where this cable leads and which part of the power robbers could be! thank you and many greetings Tommy