If it is from an airdrop, then the item disappearing is not an issue. The nature of the airdrop is that it’s limited - it’s limited to the first few people who claim it and it’s limited that you can only claim it within a minute of its drop. If you have not claimed it by this time, then the item would disappear.
There are also instances where you would open an airdrop crate and it would expire before giving you an item. This would be due to the timer running out right before you opened the crate.
Sliver will not return expired items, but we do wish you luck in the future airdrops! Airdrops happen frequently.