Yeah, the reason Petra currently needs to see the entire screen is because it decides when to launch an attack based off of the current number of buildings, units, etc. that it's target has, as well as where they are. The hard part would be naval maps, where it would have to sail around with a ship (which it currently only builds fishing boats unless it is attacking) until it found your base. And, on maps with large islands where you are not visible by a ship sailing around you, Petra would have to be coded to land on an island, send soldiers to find you, then get back in then ship and sail back. Another reason Petra can see the entire map is so itcan change it's plans based off of what has happened since it launched an attack, i.e. to make sure it doesn't just walk past new buildings that weren't there when it ordered the attack.