My 2001 VW Jetta is leaking coolant--not a lot, just a slow leak. Any ideas what could be going on?

Nah, that is internal(so it would leak internal-meaning thermostat is not cutting off the coolant supply to the motor when cold 100%>which is not a big deal<and you would never ever notice even if you were physcic. "O" ring is like a gasket; it just lays there. . Try something even simpler. There is where the spare tire is a repair bag which has your wheel socket and straight bar and a screwdriver which has a dual end(flat and Phillips) Use that screwdriver and tighten all the hose clamps in the engine compartment. They may move another half turn. . Also get down on your knees, and open the drivers door and in the center right behind the gas pedal is2more clamps which go to the heater core. If the carpet is wet right there, then they be the leakers. Leave heater core alone. That is fine. Front passenger under the car you will see the waterpump. Lay your head on the ground and look up to it. There is a small hole at the bottom of the pump. If it has a red stain or is wet with coolant, then waterpump needs replacing as the waterpump seal is worn out. If that is dry and just has road dirt on it, it is not the problem. Then one of the hoses has a pinhole leak which only causes a problem when engine is hot, or the tanks(either the top tank or the overflow tank has a crack in it. Feel with your fingers along its seam. This may be all you need to do to see if it is wet. Last thing is the heat control on the heater hose. It is plastic and if you use it a lot(hot/cold) that too wears out...but you should see it wet from just looking at it. Get some coolant additive and put it in and it should block the tiny pinhole leak.

Go to VW Junkies forum and ask there, but yeah check your hoses and your radiator cap.

turn on your car and turn on the ac full blast and get the car to heat up having the ac on will help heat up the engine, go look under the hood when the car is at normal running temperature and look where the fluid is leaking from if its a hose then it should be more the 40 dollars if its a radiator leak it could cost up to 250 dollars because its a Volkswagen! Check for leaks and determine what you will need! DO NOT USE the radiator repair gel that you pour in it is very bad for your cooling system! Hopefully its just a hose!

look for the leak hoses and fittings