The frightening effect of the advent of the Nazgul - and it is said the shriek of them was the thing men had to fly away from.
Every upgraded police-force - and some ships sailing near the Somalian coastline - have these sonic weapons which force people to back away in horror as the sound just feels like it bursts their brain into pieces.
Elvish blades that sense orcs - any modern sensory equipment that senses movement at certain radius and/or scents. The orcs would actually smell something of their own and it shouldn't be hard to built a sensor detecting that strong a smell today.
