Air switing valve. Part of the evaporative emissions on CA models. Someon's probably removed it if it's not there and your aren't the original owner. Look under air cleaner on the Revzilla OEM parts diagram.
Don't leave it open cap it off somehow either with a rubber plug/cap on the titty of the aluminum cap after removing the hose or with a block off plate and eliminate the aluminum cap altogether it looks cleaner to me personally
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