Weird error message on S3 Frontier

[Mike-Mike]

I took my S3 Frontier (T-Mobile) off to go to the gym.

When I came back I had this error on the screen

 

[Mike-Mike]

After a reboot it seems fine.

 

[lyingfromyou]

Kinda blurry, what does it say??

 

[Darth Mo]

Kinda blurry, what does it say??

"This phone has been flashed with unauthorized software & is locked. Call your mobile operator for additional support."

 

[lyingfromyou]

Oh wow! Has it done it since? Have you tried to root it recently or ever?

 

[Mike-Mike]

No, I never rooted or did anything.

It has not done it since