Light leak? Reflection? Something else? Zenit-S

So that's the problem. It only happens when camera is held this way. If I turn it 180 degrees to take a portrait-form shot "the other way", it does not appear. Neither it does when taking a landscape. Haven't tried a landscape upside-down, though. Looking inside the mirror box I can't see anything obviously reflective, and the curtains look just fine. Any idea where I should look next?

Check the film door seals on the left side.

This camera does not have a film door, only a detachable bottom plate.

Thanks to a private message from an APUG member, I've found the cause of leak. The light comes in through the viewfinder.