The plenum is another drainage hole on the outside of the car, which when blocked can damage the ECU. There's a how to on modifying the plastic panels under the windscreen so that you can easily check it for blockages here:
Just to clarify David, the plenum is the void between the engine compartment & the bulkhead, the plenum drains are the tubes found on the inner wing & more importantly, under the ecu & pollen filter at the base of the plenum
https://www.the75andztclub.co.uk/for...d.php?t=125864