I will be checking out of an AirBnB in Antibes and likely arriving in Menton prior to the check-in time for my accommodations there. Neither of the train stations in Menton appear to have luggage storage available, as far as I can find out. With some Googling I found Radical Storage, which purports to operate in cities around the world. Has anybody had any experience with Radical Storage?
Not Radical Storage but I have used Nanny Bag in France. They operate through various hotels, small shops, etc. Ours in Paris near where we were spending the day before our evening flight was in a small 2 star hotel. That hotel had a couple dozen bags in a locked closet they kept. Worked great. We did have to book and pay online in advance. You couldn't just show up.
Have you checked with your hotel to see if you can drop your luggage off prior to check in? Many hotels allow that.