That’s a littler harder to figure out, mainly because I don’t play Rogue often enough to have a clear experience from it.
However, from what I have seen the AI reacts in much the same way you would. As in, I believe they ‘notice’ the unit, but once it goes invisible again they lose track of it, much as you would. Have you ever had those pop ups telling you a unit has entered your domain but when you click on it, it takes you to a random location and the actual screen showing what the units were is blank? I’m pretty sure that’s due to concealment and it may well be the same with the AI. They know something entered their lands, they know approximately where at, but they don’t have specifics.
For example, in one game I was using a stack of assassins camped outside the capital of an enemy and I wanted to get from the forest, to a resource node closer, and within one turn of the city proper, to do so I had to run across open fields. Undoubtedly the AI saw this, but they did not send troops to investigate nor did they begin to reinforce or send troops from elsewhere to fortify the town. Indeed, a few units actually left the city some turns later and I struck from the shadows taking the city with relative ease.
In another case, using forest concealment again, my army was forced to run through open lands near a city. This was a considerable sized army and in response the AI started to fortify their city and move units towards it. They didn’t try to tack the unit down, as it was hidden again, but they did know a force capable of taking the city was hiding somewhere.