ChatWatch

ChatWatch is an app that lets you monitor the phone numbers of your WhatsApp contacts. You can see their last connection time, their most activity time ranges, their first connection time of the day, and much more.

Another surprising feature of ChatW lets you compare the connection times of two or more numbers. Like this you can get an idea of whether there are recurring conversations between these two contacts. The question, of course, is why in the world you would want to. What can you gain from that sort of action? Probably nothing. And the most likely thing to happen if any of the people you're monitoring find out what you're doing is they won't want to have anything more to do with you.

ChatWatch is an app whose aim is at minimum questionable. Plus it has weekly or monthly subscription with some bonkers prices. That said, everybody is free to do what they like with their time, money, and personal relationships.