since the ability detect mine, makes mines lose their invulnerably, you could give the mines a huge health regen rate and amount of health so as they are nearly impossible to kill, there are other ways but that is the simplest one I can think of at the moment.

All the things you could do to make an object harder to notice.

1. Remove the icons and string. This will remove it from the Empire tree and HUD.

2. Make its ability use the Invisibility modifier instead of phased out used by unyieldind will to make the mesh invisible while it is active.

The problem is both of these things will affect both players equally, and the AI might still know it is there.

Actually not. The AI doesn't see like we do, for example one trick for manipulating the AI is to place research off the tech trees so no human players can physically press the button to research them, but the AI still can. Likewise the AI likely keeps a list of all objects a in gravity wells it has explored, even if there is no visual clues for human players.

To just make it so scouts don't reveal mines, just remove the ability from them. Though that will mean the only way to get rid of them will be to detonate them...