Mini Cooper coolant bottle losing water

Coolant bottle losing water slowly for the last month, garage has been monitoring level on short journeys and long journeys no visual leaks hearing gurggling sound in dashboard when sitting at junctions but have not heard for a while now. Ran engine and watched level of water rise and bubbled. Diagnostic tested for water in oil but none found. Garage want me to monitor for another week then take back. Garage have told me it might be the head gasket.