Coolant level slowly drops with no puddle

Start with the test that can prove you wrong. If a shop can't duplicate it on the 2006 Mazda RX-8, bring a video plus freeze-frame data, not just a story. That's the branch I'd chase this week. Stop there until you know. Happens in gear, not in park, for what that's worth.

Tried a second charger / second scan and the story on the 2006 Mazda RX-8 didn't change. Checking again after work.

Intermittent stuff lies if you only catch it once. Capture voltage while the 2006 Mazda RX-8 is actually failing — after it restarts you've lost the clue. That's the branch I'd chase this week. On “coolant level slowly drops with no puddle,” that was the fork that mattered for me. I'd want that in writing, not as a promise. This is the filter I use when the thread starts repeating itself. I'll update if anything new shows up.

Ask what changed right before it started. Move a coil or plug on the 2006 Mazda RX-8 if it's cylinder-specific and see whether the code follows. That's the branch I'd chase this week. Take it or leave it — that's what I'd do. A day of patience here is cheaper than a wrong part. I'd rather look cautious in this thread than stuck later.