How to determine where this memory leak is coming from?

Answer 1

It looks like you're trying to use NSZombieEnabled and leaks at the same time. These two diagnostic techniques don't work together--NSZombieEnabled makes all deallocated objects just stick around, so they will appear as leaks.

Simply turn off NSZombieEnabled and you should be set for this case.

Answer 2

As Joey said; you can't use Zombie Detection and Leak Detection at the same time.

You have Zombie Detection enabled in the Object Alloc instrument.

Turn off zombie detection and Leaks will work again.

The Leaks instrument really should detect this situation, thus I filed <rdar://problem/7139389> and have included a link to this article in the bug report.