It starts in the deep end of the pool which is facing towards the east., and the shallow towards the west. This happened when it was sunny and about 90 degrees. however it has recurred in different temperatures and times of day. The line looks like what anorlunda referred to, but it is straight...
I hesitate to say this is an anomaly, as it probably has a simple answer, but I have noticed a line of light moving horizontally across my pool every now and then, it starts at the deep end and moves continuously to the shallow end, moves up the wall at the shallow end, and ends at the surface...