how to create test strategy for additional functionalities which are not mentioned in requirement?

Can anyone please explain how to create test strategy for those functionalities which are additionally added in our application & that is not mentioned in requirement doc.

For example - there is one functionality of button click that on double-click that button will active & another button will active on 3-clicks, On managing such functionality developer missed, buttons are now working with 5-clicks also.

So should we plan any proper test strategy to find these types of additional test cases? If yes, then please let me know.

Thanks in advance.