A guard post is a massive structure. Why would it be invisible?
Immunity to EMP could help, because that would prevent passing enemy units from automatically idleing those guardposts. That would have to be limited to starflare and supernova guardposts though.
EMP immunity could be explained with those guardposts using only very simple electric circuits. That would mean, we disable the friend/foe detection and let them detonate on everything that comes close.
Still, if a large group of enemy vehicles passes, they will range-attack the guardpost and likely destroy it, before they are in its detonation radius. Multiple supernova guardposts close to each other may work better, but they are also more expensive.