After reviewing the Scenario Editor's Manual (which I relinked in the stickied post), and thinking about raiders a bit, I remembered a few things.
First, when you make a site on the World Map, you can select the checkbox "Spawns Raiders" for that site. That performs no function for the game, other than providing a graphic on the map that indicates to the player that the raiders are coming from that city.
It is therefore possible to send raiders from any city you want, because the World Map plays no role in determining this. So sending raiders from hidden sites should be possible.
To spawn raiders, you merely need a functioning trigger, with at least one valid Condition (which can be evaluated as TRUE), and at least one Event (which must be of the type Raiders_X, where 'X' is the type of raider, e.g.
So without seeing your scenario (I'll be happy to look at it if I can download it), I think there must be a problem with your trigger.
The simplest trigger would be something like this:IF Condition (Player looks at me funny) THEN Event (Raiders_Greek).
This will spawn 4 to 5 raiders from a random map entry point and they will come in and wreak havoc, as much as 5 raiders can, anyway.
To have a larger group of raiders, you need more than one Event in the trigger. Many of us give, say, 4 Event objects, such as:Raiders_Greek AND Raiders_Greek AND Raiders_Greek AND Raiders_Greek.
This will produce something like 16-20 raiders, which is a little more problematic. With enough raiders stealing enough goods, you can create significant dissatisfaction in a city.
There are a number of ways to make this trigger fail to fire.
The first, and simplest, is to have the trigger set to OFF. This will mean the trigger will never fire, and raiders will never spawn, unless another trigger first sets the raiders trigger to ON. Many of us have mistakenly clicked that checkbox and wrecked our triggers before, so check that.
The second would be to put in some Condition that cannot be met, like requiring the player to open one of the hidden World Map sites, which would be impossible, of course.
These are just a few ideas to check.
Let me know what you find.