Coolant level in radiator slowly dropping?

Routinely checking radiator coolant level, I always have found coolant surface about one inch from radiator filler ring. Now, however, the coolant surface is about 2.5 inches below the same filler ring.
1. Of course, I have checked for fluid drip marks in the dirt surface below the radiator, but have found none, so far.
2. The drop in fluid level could indicate a loss of coolant into the engine cylinder(s). On that possibility, I have watched for an actual increase in oil level (due to coolant entry) but have found none.
3. Oil level is normal but drops very slowly as typical for an oil burning engine. Have checked the oil for smell and appearance, but oil seems normal.
4. Plan to remove the coolant reservoir and trace the overflow tubing from radiator to reservoir for leaks / signs of leakage.
5. Have checked beneath the water pump for weepage, but found none, so far.

I plan to place a large cardboard panel beneath the engine to show the general location of any coolant drippage.

On this 250k engine, are there other items I should check?