I found an issue with the 1.5.0 edge API where the getLabelPosition will return 0 if it doesn't find the label name. This is the current description for the function.
Description: Returns the position of the label aLabel in the default timeline, or undefined if aLabel is not a label on the timeline. Behavior is not guaranteed if aLabel is not a string.
Since it doesn't return undefined, it makes it impossible to have logic that tests for symbols that don't contain the label. Currently, the only work around I can think of for this is to modify the minified source directly.
Thanks for the report. I've filed a bug with our team to get this fixed.
If you already have one or more labels defined in the timeline, you should get undefined returned after you use getLabelPosition() to call an undefined label.