Mars in 12th can have a secret talent for hiding motives, your motivations and actions seem different from what they actually are a.k.a concealing. Spying perhaps?
Alternatively, bringing out the hidden details of conflicts to resolve things.
Be careful though because the 12th also relates to our own neglect and ignorance
I actually heard that the 8th house relates to our secret powers, maybe power and talent are different?
For example, i have Gemini 8th, and i'm really good at jumping from different styles or pieces of information and knowledge to learn more about how the world works, i also have noticed throughout life enjoying the study of meta-learning, learning how to learn, this is something i feel has helped me gain a lot of insight, and insight is a nice power. The things i learn however i do not flaunt carelessly, so it's kind of a hidden possession?