If its missing you have most likely got an error in the code or the markup .... But you should really never alter core code ... Instead you should implement these sort of changes in a custom module

I think you may be able to 'extend' the default membership provider and override the ValidateUser method. This method returns a boolean value indicating if username and password supplied was correct. You could do additional processing before returning from this method.

First step is to write a new class extending/inheriting from SqlMembershipProvider (if that's the one set in your web.config) ideally in a separate library project. Next in that class, override the ValidateUser method and implement your checks there. Finally, reference your new provider class as the default provider in your web.config as: 

<membership defaultProvider="YourProvider" ...>

<add name="YourProvider" type="[class], [assembly name]" ... />

</membership>

My recommendation would be to add minimal new functionality and reuse what's already there.