My doubt is ¿"last seen" indicates that the user has actively opened the applicaction or that the application have been running on background connecting to WhatsApp servers? The WhatsApp FAQ now says:
"Last seen at..." refers to the time the contact left WhatsApp
But googling I found an old WhatsApp FAQ answer for the same question, maybe referred to the old versions:
"Last seen" simply indicates the state of the device network connection and not if the application is actively in use. For example, on some operating system such as BlackBerry and Android, the phone will connect on it's own if it has a new incoming message even if the phone is not in active use at the moment
I have seen "improvements to "last seen" and "typing..." correctness / timeliness" in the 2.7.4705 version (2012-MAR-05) changelog
I will test it
WhatsApp uses push notifications (the server notifies the device) so seems not neccessary connecting periodically to the server looking for new messages (pull). Another question is if once a notification is received the app connects to the server for new messages downloading. In any case, the question is if WhatsApp takes into account the connections with the app in background for the "last seen" indicator.