Bill, I just found this effect you thought of reported here by Stephen and is called the Gamma-Lamba effect . The magnitude of my results correlate closely to those reported by Kodak for Tri-X.
Originally Posted by Bill Burk
In fact UV photography suffers from this effect and when I photographed the white towel it would have had optical brighteners from my washing liquid (that fluoresce UV) and possibly further amplified the effect
What a cool serendipitous discovery I stumbled upon in my testing.