Offscreen attack notice is a method of showing if your units are attacked outside of your visible area (the part of the map that you can see at any given time). Thus, it is helpful if a stray unit is attacked when you aren't paying attention to it, or if your base is attacked when you are commanding another group of units, etc. This is controlled in the faction XML.
<attack-notice enabled="true" min-delay="15"> <sound-file path="sound/attacked.ogg" /> </attack-notice>
This is the main body tag. Enabled tells whether or not to use attack notice (if you aren't going to use it, you can just forgo even having the tag). Min-delay is the minimum number of seconds between attack notices, so as to prevent you from getting spammed with them during a large scale attack. If the min-delay is too high though, the attack-notice may not fire when you are attacked if you have already been recently attacked.
There can be as many sound-file tags as you want, as each will define an individual sound that can be played when the enemy is sighted. One will be randomly played each time. The path is the path to the sound file.
Differences in versions
In GAE 0.3.x, the attack notice was sound only, but in 0.4.x, an image showing where on the minimap the attack is is added.