The invention relates to a fuel retention monitoring system for a pressurized hydrogen storage tank on a vehicle. The vehicle includes a hydrogen dispensing system having a plurality of conduits for carrying a hydrogen gas. The conduits have at least one connection linking them together. The monitoring system includes a vapor concentrator for the hydrogen gas, shrouding a first one of the connections. The first connection has a parting line where the conduits forming the connection meet. The vapor concentrator may be positioned above the parting line of the first connection and may include a periphery and a top defining a headspace cavity. The monitoring system further includes a hydrogen gas sensor locatable adjacent to the headspace cavity and capable of providing a signal correlated to a hydrogen gas concentration in the headspace cavity.