Instead of typing this out from the start, I'll copy and paste what I originally sent to Steam Support 9 days ago (that still hasn't been replied to at all):
"For the past few months, there have been items mysteriously disappearing from my friend's Team Fortress 2 backpack. I have used backpack histories from "www.tf2items.com" to track down the missing items when they still existed. Using the individual item ownership histories, I looked and saw that she was still the most recent owner of the items. So there's no way they were accidentally traded away, or stolen. According to the ownership histories, they SHOULD still be in her backpack, but they simply aren't. She uses Steam Guard and has not given out her login info to anyone, even me. She uses a Mac laptop, and also uses Steam mobile on her iPod Touch.
The disappearances always seem to happen on days of either a TF2 Update or a Steam client update. And then they're just gone completely. They usually seem to disappear when she's not even online. There was another scare like this today, around the time of the Steam client update. I went to check her backpack to make sure the items I'd crafted for her last night were still there. Of the four new items I'd crafted (all at nearly the exact same time), all of them were there except one - a level 99 Familiar Fez. We were excited about having one of a better level to replace the one I'd previously crafted for her. But it was nowhere to be found and its spot in her backpack appeared like this:http://imgur.com/f6d9MQQ
Luckily, after the Steam and TF2 item servers came back up, it was there again. But usually we aren't this lucky. A lot of items have mysteriously disappeared without a trace and never came back. Most of these didn't have much monetary value (other than a Genuine Dragonborn Helm), but they did have a lot of sentimental value. As well as some of her Genuine promotional items from the Total War bundle, that she took pride in being the only one listed in the ownership history of. So it has been very upsetting.
What could be causing this, and is there any way to keep it from happening again? She's lost several of her favorite items this way, and frankly it's just not fair."
This has been happening on and off fairly consistently since late November or early-to-mid December. It also happened last summer (with a few of her high-value stranges that had thousands of kills, as well as hats and miscs), though less often.
I've been checking her inventory regularly to try to keep an eye on which items were still there and make sure they don't end up missing.
It happened again today with the Mutton Mann #740 I'd crafted for her, her lime Your Worst Nightmare, and a Russian Rocketeer I'd also crafted for her. They were there this afternoon, but when I checked back later they were nowhere to be found and their item histories stated that they SHOULD still be in her backpack. This has once again made her very upset, as she loved and regularly used all three of those items.
Her backpack has never run out of space, so that is not the issue.
Items have been mysteriously disappearing from my friend's backpack on and off for months, usually on the same day as an update. Not hijacked or stolen, item histories state they should still be in her backpack, though they aren't. Very upsetting for her and I both.
Has anyone else experienced anything like this?