In the English version of the game, the description for Armed Guards reads: "Armed guards can aid in keeping suspicious individuals away from secure locations. However, the very presence of the guards may itself invite greater suspicion.".

However, a quick look at items.dat shows that Armed Guards are twice as effective as a Perimeter Fence with no downside save higher cost and longer build time. On my first playthrough of Singularity, I thought from the description that Armed Guards would decrease some detection chances but would increase others, perhaps the Covert chance. Since I was mostly being detected by the Covert group for most of the game, I never built any Armed Guards even when it was a good idea.