I'd put money on the heater pipe o-ring at the back of the water pump. The o-ring seals when pressurised, but once it cools right down, starts a small drip. If you had green coolant in there you would see the tell-tale salt building up around the leak, but sounds like its probably running on near straight water by now. Fix it and get proper coolant in there, before it rusts out and you blow a head gasket or something. Can happen pritty quickly.
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