Mont St Michel

It is not unusually dangerous to do this but you always take a fair risk when leaving luggage in a car; I know someone whose car was emptied while they swam at a beach in Italy. I think there are lockers in the parking lot at MSM where luggage can be checked. You might want to see if that is the case. If you do decide to do this, then don't open your trunk in the lot but have your bags ready in the back seat so that you don't open the trunk to retrieve them and leave nothing showing in the car. The lots are enormous; it would I think be fairly easy to rob a car.

Last summer we met our friends in MSM; they had just come from Ireland on the ferry and drove direct to MSM. They left all their luggage (family of four) in the trunk of their rental car. Nothing was stolen. I think you will be fine if the luggage is in the trunk of the car; and yes, do not open the trunk in the parking lot and expose what is contained there. I would take your passports or medication or anything truly valuable out of your luggage and take that with you. I really think you will be fine leaving your luggage in the fully hidden trunk of the car.

I have been to MSM twice; it get better every time I go-I adore it!!! If you have time, do walk out on the mud flats at low tide; it is wonderful! There are low water faucets, near the entrance of the walls, to wash your feet when you get out of the mud; I'm sure others will go there, just follow them.

The cathedral at the top is magnificent!

Note that the hotels have their own parking lot - perhaps it's even free (we didn't spend the night. And I've never been in the "hotel" lot.)

http://www.bienvenueaumontsaintmichel.com/en/preparing-your-visit/arriving-by-car

That said, you're asking for an estimate of risk. Not a guarantee that nothing bad will happen. I'd add that lots of Euro rental cars only have a windowshade-like roller over the luggage, and the outlines of the luggage can often be seen.

Tim is correct. The hotel you will be staying at has assigned spaces close to the entrance with no risk of flooding and that parking area is monitored. And yes, you do not want to drag too much to the room if you can avoid it. Rooms are small and walls can be thin, but the ambiance more than makes up for the shortcomings.

So much for the basics. Evenings there are fantastic. We've dined at the Mere Poulard (overpriced, but worth the experience) a couple times and loved it. After the wine and/or champagne, you can enjoy an "illuminated" stroll, sans crowds, around the island. Be sure to make your escape before the buses arrive in the morning, usually around 9am. For an offbeat experience, if you're into history, within sight of the island is a German WWII cemetery at Mont-de-Huisnes. A bit creepy, very moving, and worth a few minutes of your time.

Enjoy!