I honestly cannot recall leaving anything of value anywhere. DD15 on the other hand, I have a few.
We bought a fancy new camera a few years ago. Our first trip to WDW with it, I told her if she wanted to use it she was carrying it around. On our HS day we stopped first at Starring Rolls for cupcakes for breakfast before heading to TSMM. Right as we were getting in line for TS, we both realized she'd left the camera bag on her chair. I told her to run and get it! Thank goodness it was still there. Pretty sure DH would've had our heads on a platter. She said there was a nice couple that noticed it and if she hadn't gotten there when she did, they were going to take it to guest services.
My grandma gave DD a bunch of her old costume jewelry. There was one ring that she loved and wore it every where. Shortly after DG passed away, DD left the ring in the washroom at my oral surgeon's office. We were 20 minutes away before she realized it and started panicking and crying (while I was driving, nothing I could do). I got on the phone right away and they said it was still there, so I went and got it the next day.
Not so valuable, but 2 years ago at the end of our WDW vacation, we stopped at a washroom before security. DD took her sunglasses off of her head and set them on the toilet paper dispenser. As soon as we got through security she realized it. I told her they were long gone, I was not going back through security. I still give her grief about that one!