About 1.5-2months ago I changed my thermostat. Lost a few litres of coolant. Topped back up, drove for a few weeks no problem. Then got a coolant warning light. I pull over and check and the reservoir is empty. Top it up, no foamy stuff in oil filler cap, go on driving. Put cardboard under the car overnight - no drips/leaks, no foamy stuff in oil. All seems well. Today, I start up and go 1/2 a mile and the coolant light comes on again. I pull over, reservoir is empty again - no problems in oil, I top it up and all appears fine. No leaks (put cardboard down all day). Anyone have any thoughts on what could be causing this? I don't really have any experience in looking for leaks. All I can think is it's a failing reservoir cap or a small crack in the reservoir bottle (perhaps?) If it's relevant, I had the water pump changed 2 months ago (when the cam belt was done) Any ideas appreciated!