The Microsoft debug libraries have some memory leaks in them.  It’s frustrating but I’ve given up on making my debug builds leak-free.  If you’ve inspected all your DEBUG-conditional code carefully and you’re sure there are no leaks in the release build, go on to another problem.

I suggest you introduce an intentional leak to make sure leak detection is working properly in your release configuration.  Then write off any leaks that occur only in the debug configuration.