Usually, when intentional backdoors like that get found and fixed, the 'someone else' stays silent. Otherwise, they provide proof that they've been planting backdoors, and that's much worse than having a hole plugged.

To get an idea of how this stuff usually works, start with the Simple Sabotage field manual:

https://ia601309.us.archive.org/14/items/Simplesabotage/Simp...