You could use the user_logged_out signal combined with the messages framework:
First, make sure the messages framework is set up (https://docs.djangoproject.com/en/dev/ref/contrib/messages/).
Then include this code somewhere that will be called (I tend to put it in a receivers.py module and then import it from a models.py file in an installed app):
from django.contrib.auth.signals import user_logged_out
from django.dispatch import receiver
from django.contrib import messages
@receiver(user_logged_out)
def on_user_logged_out(sender, request, **kwargs):
messages.add_message(request, messages.INFO, 'Logged out.')
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