Losing small properties is part of the game.

Left in a car while hitchhiking

Sweater
Cap of thermos flask
Gloves
Winter cap
Map of Germany
Map of the Netherlands
Map of Belgium
Map of Poland
Several cardboard signs
Mobile phone
Travel schedule (phone numbers, addresses)

Lost my mobile phone and travel schedule on the same ride. I’ve written about it. That was on a ride from Vienna to the German border, end of A3. I called my phone a number of times but the phone was not answered. When I arrived at my host in Würzburg it appeared the driver had called my host there, with the message also that he wanted to return my mobile phone by post. Few days later I read that he had also called one of my other contacts, a friend in Poland. Also lost my travel schedule, but I was almost back home.

And for comparision: lost in train while travelling

Very good tent
Mobile phone
Flowers
Umbrella

Both objects did not come back. For the tent I contacted the lost and found and ordered a research, without result then.