2001 v70 with 96,000 miles. Twice in the last year, the coolant light blinked on and I found the reservoir below the minimun level, I added about 12 ounces to fill the reservoir and all ok for about 6 months. No spots, obvious leak. No odor of coolant anywhere. Any ideas?

 

Not really. You need to check it more often. If you don't see a leak keep watching for one. It will likely get worse.

If you have a minor head gasket leak the only thing you can do is wait it out to see if it gets worse. You can also check your plugs to see if you see anything strange there.

I think our S70 goes through about 2 oz of fluid per month.

 

PS. When my resevoir drops, I check it every drive cycle ; )

 

I'm assuming its an n/a car. Regardless, with such miniscule loss, might be leaking around somewhere out of view like lower radiator hose/ block elbow. Rust and scale form there over time and minute ammounts can easily escape.
Rspi is right. Check more often. I use a felt tip pen to mark the degas bottle on mine to keep an exact eye on it rising and falling.