"remained secured" is correct and usable in written English.
You can use it when you want to indicate that something has stayed in a protected state. For example, "The perimeter of the mansion remained secured throughout the night, ensuring that nothing was disturbed."
Only the first office voicemail remained, secured these three years in its electronic vault, the combination encrypted in Julia's private set of lucky numbers.
Even when Arcade Fire closed out its set with a New Orleans-style parade through the crowd — daring the masses to channel their inner Mardi Gras — things remained civil.* Thankfully, no dangerous beads were thrown and all breasts remained secured in clothing.
The baby remained secured in her car seat until bystanders pulled the child out of the wreckage and tried to perform CPR.
The ties that bind remained secured as Miller Williams stood behind his daughter, the rebellious high school student who got expelled for refusing to say the Pledge of Allegiance.
The randomisation code remained secured until enrolment and data entry were complete.
An empty-net goal by Sahir Gill with five seconds remaining secured the victory.
Push or fold the wiring up into the junction box, making sure that all wires remain secured together with the wire nuts.
Harpring made two free throws with a minute left and Malone converted one of two free throws with 45 seconds remaining, securing the victory.
In 2015, Green and some colleagues completed a security audit of TrueCrypt, concluding that, the developers' shutdown notwithstanding, the software remained secure from back doors or cracking.
In spite of everything, it remained secure against loss".
For the moment Sicily and the scattered Italian possessions remained secure.
