This wiki page is a source of information about the UMBRA transmissions.
Someone on 4chan's /x/ posted the link to the Spreaker.com "Radio Station" that had recordings similar to those sent by shortwave numbers stations (the buzzing sound from UvB 76 “the buzzer”, connections to Lincolnshire Poacher), except way easier to crack. These messages were decoded and found to contain references to millenarian cults involved on acts to destruction: Order to the Solar Temple, Movement for the Restoration of the Ten Commandments of God, Aum Shinrikyo, and others. This last cult committed the Sarin gas attack on the Tokyo subways on 1995 that killed at least 7 people and injured thousands.
We know that Umbra is a mission designation, and person broadcasting is codenamed Virgil. She is sending information to us and a number of strike groups. Our team is SIFT, and we decipher these messages and communicate with a member of strike group Charon.
To Do List:
So we are interested. There is a possibility for this to be an alternate-reality game, but an interesting one.
The UMBRA Index
You can browse the content or the wiki, or you can go here for the latest transmission (#59).