Maybe part of the cruise control system? I just pulled one out of a '96 Legacy and there were two short hoses like this, on both ends of a hard line on the firewall, one to the actuator on the firewall and one to the pump/valve unit on the passenger strut tower, though I think on mine the hoses were slightly thinner and din't have any hose clamps. It seems possible there wouldn't be a vacuum leak when the cruise control isn't enabled.