Some TMNT stuff was always meant for grownups.
In the Mirage continuity, the Utromi Obscura Secreti are a rumored, officially nonexistent organization of Utrom warriors that carry out the Utrom government's black ops. They are a glorified death squad, carrying out tasks that most peace-loving Utroms would never do and would never approve of.
In First Mud (backup story of Tales of the TMNT volume 2 issue #14: Loops, Part 2 of 2), Professor Obligado uses black market information to rediscover the lost original Utrom homeworld of Magonia, but finds it has been intentionally hidden in a massive planet-sized cloak, and its surface almost completely devoid of life. Continued in Apocalypse Vow (backup story of Tales of the TMNT volume 2 issue #15: Hell's Blacktop), Magonia proves to be a not-so-lost world, as it is actively monitored by the Utromi Obscura Secreti, who are tasked with maintaining the planet's secrecy and killing anyone and everyone who dares discover it. Obligado, a trained commando, not only evades the Secreti attempt to kill and silence him, but he covertly steals one of the Secreti ships to escape Magonia. Obligado takes the opportunity to study the Secreti ship's on-board database and learn more of their secrets.
The next issue, Tales of the TMNT volume 2 issue #16: Sins of the Past, explains the impetus of hiding so much of Utrom history: It is strongly implied that Magonia was one of many worlds laid waste by the J'Gels, an alien race that the Utroms hunted down in a systematic campaign of extermination. This desperate but shameful chapter of Utrom history was forcibly removed from all official record and living memory.
In Eastman and Laird's Teenage Mutant Ninja Turtles volume 4 issue #28, Michelangelo's new Triceraton ally T'Zirk explains to him that the Triceraton Republic has many times been close to discovering Transmat technology on their own, but each time were sabotaged by acts of terrorism carried out by the Utromi Obscura Secreti. T'Zirk is not too fond of other Utroms either, but recognizes that most Utroms are indeed a peace-loving people, and that the Secreti's existence and operations are kept secret even from them.