Having 2 phones and 2 accounts I just checked. I opened Whatsapp at xx:00, other phone says "Last seen xx:00". Put phone to airplane mode, and at xx:03 I opene

Having 2 phones and 2 accounts I just checked. I opened Whatsapp at xx:00, other phone says "Last seen xx:00". Put phone to airplane mode, and at xx:03 I opened WhatsApp in that mode, then put WA on background (without "killing" it from the task switcher) and I immediately turned off airplane mode. I checked on the other phone, and it still said "Last seen at xx:00". So, it seems that line is honest, since I was on airplane mode, the network didn't "see" me.