Two Objective-C Memory Management Issues: Is Leak Detection Always Right? Why does the abstract work, but not released?

Question

I just discovered the following: As I expected, releasing my object before I return it will cause the application to crash:

   + (NSString *)descriptionOfExpression:(NSArray *)anExpression {
    NSMutableString *expressionDescription;
    expressionDescription = [[NSMutableString alloc] init];  
    for (id object in anExpression) {
    //Do stuff to expressionDescription
    }

    [expressionDescription release];
    return expressionDescription;
}

However, I did not expect the following to result in a memory leak:

    + (NSString *)descriptionOfExpression:(NSArray *)anExpression {
    NSMutableString *expressionDescription;
    expressionDescription = [[NSMutableString alloc] init];  
    for (id object in anExpression) {
    //Do stuff to expressionDescription
    }

    return expressionDescription;
    [expressionDescription release];
}

Ultimately, I decided to do this, instead:

    + (NSString *)descriptionOfExpression:(NSArray *)anExpression {
    NSMutableString *expressionDescription;
    expressionDescription = [[NSMutableString alloc] init];  
    for (id object in anExpression) {
    //Do stuff to expressionDescription
    }

    [expressionDescription autorelease];
    return expressionDescription;
}

I understand why its auto-implementation works, but how is the leak caused by the release after returning the value?

My second question is very important: are memory leak detection systems constantly detected?

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