Yesterday, I discovered that 22 items, specifically weapons, were missing from my inventory. I have had two-factor authentication enabled since I created my account and no one else has access to my PC. I have already submitted a ticket to Warframe support.
Update: I am sorry it has taken me so long to update this post. I have been really busy with rl stuff. According to Warframe support, I sold all 22 items. I apparently made 11 seperate transactions between 05/07/18 and 06/08/18. I am absolutely certain that I did not make any of these transactions. They also stated that 'there were no unauthorized access attempts and all your recent logins were under your legitimate credentials.' I don't know if this issue is limited to just weapons, or applicable to all items.