The standard behaviour for a while has been to open the action overflow (the "hidden" menu with three dots on the right hand side of the action bar) menu. This menu affordance was supposed to appear on devices that didn't have a hardware menu button.

Ah ok thanks. In that case, it does nothing for some apps! eg. if I am in the Google+ Android app and press the hardware menu button, it does nothing. Calendar does nothing. Hangouts does nothing.

If I am in Gmail, it pops up the menu (but at the bottom, and not where the hamburger is). Chrome pops the menu up. Maps opens the side bar. YouTube pops the menu up. Drive opens the menu.

So it works for some but not for others. Perhaps you could enforce consistency or follow your own guidelines etc.? Go crack the whip.

The main devices that Google devs use - Nexus devices - do not have a menu button.

And I guess they think that the three dot menu is ugly or something.

It is automatically created in the action bar when the action area is too small for buttons:

http://developer.android.com/design/patterns/actionbar.html

http://developer.android.com/guide/topics/ui/actionbar.html