Bill
More than likely, the sun shining through the lens of the photo cell has created a high enough ambient temperature in the photo cell to distort the shape of the back plane of the photo resistor. This would change the shape of the resistor pack and also the resistance. The photo cell isn't remembering the light it just needs more to make it operate.
Multiple photo cell set ups on light systems are an easy place for an electrician to sell a photocell and lighting contactor. The customer will be verry pleased with the results.
