If some of your items are missing but not stolen, i.e., you know for sure that nobody else had access to your account or computer, then it is almost always from forgetting that the items were deleted. You might be surprised how often it turns out a player deleted some items 5+ years ago and forgot about it. If you think the items might have been deleted please refer to this article: Recovering deleted items
If the item was recently purchased please refer to these articles:
Otherwise unfortunately if the items were traded away or scammed there is nothing Steam nor Unturned support can do. Please refer to this Steam Support article: My Items Are Missing or Were Stolen
Personal anecdote
If your items were stolen/scammed, you have my sympathy. It's a frustrating and sad experience, and makes it harder to trust people. I am sorry you are going through it.
When I was a young teenager, I got scammed several times on Steam, Roblox, and a couple other games. If there is a bright side or silver lining, I think you can take it as a useful learning experience to be wary of potential scams in the future. In the heat of the moment, I was angry at the people who scammed me, but looking back on it, I can be glad it was something relatively small like in-game items rather than my life savings. That being said, I of course condemn scamming and hope the scammers feel remorse for their actions someday.