It turns out that even if a player has no defending diplomatic troops in their town, it is possible to detect an enemy's presence.
In a test I did, 25 scouts were sent to another town (one of mine) 1 at a time. The defending town had NO scouts in it, and therefor it would be logical to assume that all scouts would pass undetected. however, 3 of them failed to complete the mission fully, and 1 didn't return at all. This suggests that the GMs have added a slight element to the game that allows for the detection of enemies without any defenders. However, there is no telling what it is (it might be the consulate, counter intelligence research, or other diplomacy units) and it is certainly not a good idea to put any trust in it, as 4 out of 25 units failed, and each mission only had 1 unit. 100 units would have a much better chance, their odds would increase 100fold, so in 25 missions .04 would fail. Do 2500 missions, and you might lose 3 or 4 of them.