I checked the condensate drain line and although I could not see it, I could feel the last 2-3 inches of it. The line was soft and flexible and appeared still be attached to the fridge - I could pull on it and it would stretch a little but seemed to be firmly attached.
However, there was no apparent "P" trap in the tubing in the amount I could feel and even if it has a trap in it someplace, I am not sure if the fridge is used enough that the trap would be full of water. So it is certainly a potential source of cooling loss for the fridge, but it's ID is only about 1/4 inch so not sure if the loss would be enough to account for the results I see. But it is easy to plug on the inside of the fridge to eliminate this as a possibility.
Thanks for the suggestion.