Good question. I assume you're referring to satellites? First: they don't cover the whole globe - in fact their passses aren't that hard to figure out. In his book "See no evil", former cia operative Baer indicates that the Kurds were adept at moving and attacking "invisibily" with quite large forces during the gulf 1 conflict. One of the reasons he left the service was that Langely didn believe his reports on the ground: claiming the satellites hadn't seen anything: he must be wrong. (This was when the focus was on gadgets and not HUMINT, afaik CIA have gone back to using more human sources again, having been made fools of by Al Qaida etc).

Second: we don't know much about what anti-satellite weapons the various powers command.

Edit: also submarines can't be detected from space