Only way for coolant to be “consumed” is via a leak. Leaky hose, fitting, seal, pump, cooler, o-ring, heater core, or head gasket.
If your not the type who fixes your own find a recommended independent repair shop that is tdi savvy and get it sorted out.
Coolant is not supposed to be a consumable fluid ala engine oil. Dealer has clearly demonstrated their incompetence and lack of troubleshooting skill by saying it’s not leak. It’s definitely a leak; they just can’t, or won’t look, find it.