Be sure to take the hardware for the triangle. Some people have forgotten that or left it on the car. It is a perfect place to mount the Euro plate when you get back home.
Be sure that the first aid kit is in a checked bag. There are some sharp things in there that won't make it past security.
For what it is worth, I left a couple of things I didn't really care about in trunk and they made it back. I have heard about some people leaving dirty laundry in there that made it back.