For most context menus, the Target attribute contains both the selected objects (if any) and the object under the mouse.

def MyCustomMenu_Init( in_ctxt ):
    oMenu = in_ctxt.Source
    oMenu.Filter = "camera"
    oMenu.AddCallbackitem("Custom Camera Tool","MyCameraTool")
    return true

def MyCameraTool(in_ctxt):
    obj = in_ctxt.GetAttibute("Target")
    Application.LogMessage(obj)

I’m pretty sure I’m the one who dug up that info and put it in the docs (on the Menu Item Callback page). I did not, however, write this bit, which to my ear does not sound good at all:

If your menu is attached to a contextual menu, the currently selected objects are passed in to your callback. The target object under the cursor is also passed in as part of the selected objects. However if no objects are selected, then only the target is passed in. The objects can be retrieved through the Context.GetAttribute method with “Target” specified as the value for the AttributeName parameter. The selected/target objects are not passed in to the callback of a custom menu item attached to a regular menu.

I prefer something more like this:

For context menus, the callback gets the currently selected objects and the object under the mouse. You can get these target objects from the Target attribute with Context.GetAttribute.