I've mentioned this on a different thread, as the OP (with a cheating husband) had suggested that his WhatsApp was easier to check for ilicit messages than Snapchat, because SC has disappearing messages (although WhatsApp also has this feature, it's just very conspicuous and takes a bit longer for them to disappear I think).
But WhatsApp has an even better feature than disappearing messages for cheaters called 'Chat lock'. I told my fairly tech savvy friend about this recently and she was shocked when I showed her how it worked, and she didn't know about it at all. I'm wondering how many other radars it's gone under.
Chat lock can create a secret folder for messages which is not visible in the app anywhere at all, and whether the feature is being used or not is not shown either. It is password protected or biometrics. You type the password into the search bar at the top of WhatsApp and a little icon with a lock appears, it unclicks, and you tap on it to enter the secret folder.
As I say, there's no way of knowing whether this feature is being used if it's been hidden, and if you try to change the password in the settings to get into the folder, then all the chats get deleted anyway. The only way to make the folder appear is by knowing the password... Which, if the user is clever, will be a long string/pattern of every day emojis that won't look out of place in the recently used emojis.
The only way I've figured to see if the features being used is if the contact that's been hidden is saved in the phone. If you go to the phones contacts, find the contact you're suspicious about, and click 'message on WhatsApp' - it will show that the chat is locked and will ask for a password. So you could get proof that something is being hidden between two people, just not exactly what it is. That only works for numbers that are saved, though.
Anyways, have a look for yourselves if you don't know about it, you'll find it in WhatsApp settings.