I have had a problem since my last service.

Hi, I have had a problem since my last service (then 60000) where the coolant level has dropped slightly, enough to set of the warning light on the dash. We kept topping the reservior up (only a small amount). This had occurred 4-5 times during that 15000km period. Just had it serviced, got the mechanics to check it out, they said they pressure tested and checked it out and could not find a fault. I have just gone out to start my van after 3 days stationary, and the light has come on again, and there is a slow drip coming from underneath the engine. I cant see exactly where from, and Im no mechanic, but its definantly radiator fluid. Before I take it back to them again, I was hoping someone may have some usefull information into this probem. Any help appreciated. Thanks.