The 3.1's have a known defect with the lower intake manifold gaskets that allows a small amount of coolant to be drawn into the motor and burned. At the rate you are loosing it, you'd notice nothing in the way of exhaust steam.
You might be able to see evidence of it on the affected cylinder's spark plug, but I doubt it. That's a very minor loss.
It could be worth adding stop leak. I personally use Alumiseal, in the little tube, finely ground silvery powder. It works well on very minor internal leaks like that.
Once you add a product to it, never add more, or you do run the risk of hogging up the system. A lot of people don't put the product in correctly, and that can be real bad too.
If you use it, do this.
Remove the RADIATOR cap (I've seen people put it in the overflow resevoir, really!) and start the car and let it run until full operating temp is reached. Observe the coolant flow, and when the thermostat opens and the coolant is circulating strongly, bring the revs up slightly. This will make the coolant drop in the radiator slightly, dump the stuff in all at once. Hold the throttle until all the powder is circulated, then put the cap on before dropping the throttle. Have someone help you if you aren't dextrous enough to do it all at once. Then drive, drive, drive. People put the stuff in, put the cap on, and shut it off. And the sealant turns into a big wad in the radiator. Driving at least 20 miles will ensure even distribution, and give it ample time to be drawn into the leaking area.