According to a post on its website, Tether has experienced a sustantial hacking event with a huge loss of about $30 Mln from the treasury account.

"Yesterday, we discovered that funds were improperly removed from the Tether treasury wallet through malicious action by an external attacker. Tether integrators must take immediate action, as discussed below, to prevent further ecosystem disruption"

This is the Tether tresury multi-sig wallet that was hacked:
https://omniexplorer.info/lookupadd.aspx?address=31okFF1rUu8jjPEVuajycTRBp82Nteo4Mv

The hacker did not only stole the USDT, but also ~5 BTC which were in the wallet for some reason.