Addendum: I verified flag 8 is Mod1 is Numlock on Windows.  http://infohost.nmt.edu/tcc/help/pubs/tkinter/web/event-handlers.html has table that claims that 8 is Left Alt and that 16 is NumLock. From what you say above, true on non-mac *nix, and only there.

Flag 32 is Mod3 = Scrolllock on Windows.

Question: autocomplete.py refers to event.mc_state in

    if hasattr(event, "mc_state") and event.mc_state:
        # A modifier was pressed along with the tab, continue as usual.
        return

It is not mentioned as an event field in either the NMT ref or http://www.tcl.tk/man/tcl8.6/TkCmd/event.htm#M29.  However, it seems to be the state minus any 'lock' flags, Nun/Caps/Scrolllock on Windows.  It must use a system-specific mask.  The only Google hit for "tk event mc_state" is my recent commit of test_autocomplete.py.  Is this a normal tk event attribute or a local addition?

There is also mc_type, which is 0 for a tab combinations.