Question: how to check someone last seen on whatsapp even if it's hidden?
Answer: Open WhatsDog, tap on the big Start button. ... 
Not so soon, you won't be able to see the last seen activity as soon as you add a number. ... 
To add or change a new number, go to settings icon in the app, swipe up and select 'Delete info', the app will simply start from the step 1.