Lots of great ideas being tossed around here about what the spying system should be. Here's my amalgam of them, with a few new ideas tossed in: Spies would have two settings, active and passive. Second, passive spies should be invisible until caught.

"Active" spies:
- When on other civ's planets, they behave like spies currently do, disabling or (preferably) diminishing the output of a building. If put on a planetary capital, they actively sow discontent, lowering morale and increasing your influence. Active spies are fully visible.
- When on your own planets, they behave as spies currently do, neutralizing agents.

"Passive" spies:
- When on other civ's planets, they simply spy. That means collecting intel and possibly stealing techs. The level of intel, as well as the likelihood of stealing techs, increases based on the number of passive spies acting against a civ and how long they've been there. The catch: passive spies are invisible to the civ that's being spied upon, unless they use passive spies to catch them.
- When on your own planets, they seek out other civ's passive spies. They're like your secret police. The more passive spies you place, the greater the liklihood of catching and neutralizing other civ's passive spies.

Is this possible within the current framework of DA? I don't know. Still, I'd like to hear what others think.