Ive researched some stuff about it that is quite interresting.

1. Look at date that photo was taken , then look at mission sts-088 description at wiki and nasa pages. shot was taken when shuttle was DOCKED to newly ISS after connecting Unity with Zaria.

2. Orbit inclination of ISS is 51,6 deg, altitude about 400 km. If this mysterious item have polar orbit - it was an INCREDIBLY coincidence to spot with ISS at same time when there was Endeavour.