when you dont use the air con for a while the seals in the compressor can dry out and you lose gas. if the liquid is from the aircon it will only be water and would mean the drain is blocked. but if its still leaking and you dont have an air con belt then it must be coolant from a leaking heater core or one of the hoses from the firewall to the core. turn the heater off for a bit to see if it stops, wipe a white rag on wet spot to see if its green(coolant). prob more chance of a weeping hose than the actual core.