Invisible Armor...

On Agent difficulty is a completely unseen suit of body armor on the Frigate mission.
Hidden in a room full of boxes and metal crates.
Get close enough to the wooden box on the metal crate, and Bond will collect the item.
It's also one of the rooms that can be seen briefly during one of the intro camera shots.
As far as I know, it's the only item hidden like this...

Penny For Your Thoughts...

She's being playfully jealous here.

Getting extremely jealous now.
Ladies gotta learn to share - Plenty of Bond to go around.

In The Navy...

Duncan Botwood offered to show me how a grenade worked.
I didn't think it was safe to try indoors, but he acted like he'd done it before...