iPhones are very "chatty" when not connected to a WiFi access point. They repeatedly try to find access points they have connected to before - broadcasting both their MAC address and the SSID of prior access points.
Android phones are completely silent even with the WiFi on when not connected to an access point.
Yep, this is a good explanation of it. Another thing he mentioned was that even if you have VPN enabled, the first few pings from dropbox, iMessage, etc. actually aren't getting sent through the VPN.
Android phones are completely silent even with the WiFi on when not connected to an access point.