See the video below which I recorded using a cheap endoscope.
It took me 30 minutes to find it to begin with, then even after I did, it took several attempts to be able to get back there. I couldn't catch it with a magnet at the end of the camera, but even if I did, it will probably fall when I try to pull it off.

That's where I would appreciate advice from people who already messed with that area - is there any point taking out the battery? As far as I can see there is a metal platform beneath it so it won't help me much, and I am missing a few tools to take some of the bolts out.

Or I can just try to forget it is there and ask the dealer to fish it out in the next service...

 

I had luck with strong double side tape in a similar situation. Good luck

 

Sell the car.

 

You can remove the battery tray pretty easily.

 

wow, a million to one shot there!… that would drive me “nuts” too 😆

 

Have you done it?

 

Not on my current GT4 but I’ve done it for other caymans. It’s like 2-3 bolts if I remember. It is probably one of the easiest things to do on these cars, can’t mess it up (unless you do something weird with removing / installing the battery terminals).

 

Thank you for the encouragement! Will do that first time tomorrow morning. Hopefully I will be able to get this nut out!

 

How about a claw retriever tool like this?

 

Is there an underbody panel at that location, I thought there was?

 

I did the same thing in my GT3 (dropped the nut under the firewall that retains the battery safety strap) never found it ...and I still OCD about it ...

 

You might try a pick up magnet on the end of a flex stick. Cheap off of Amazon.
Assuming the 718 has the same behind the same access panel for the serpentine belt watch the Youtubes on how to NOT drop the spacer bolt.

 

Non-OCD people simply wouldn't be able to understand the joy.

So the little bastard isn't magnetic, that's why I couldn't pick it with the magnetic tool.
Thankfully, @lowbee spirit came to the rescue!
Thanks @Le Petit Garage for the encouragement, it really was easy as you described, but it was my first time doing anything on such an expensive car, so I was a little nervous.

 

Damn, I think that you have a bigger problem, it looks like somebody stole your battery!

 

I was about to say, “Better you than I” :-) I would dream about that damn nut! Glad ya got it.