The current build, they're pretty buggy, and sometimes seem to not spawn (maybe fall out of the map), but they're there. Be careful though, as if they're not stuck on the terrain they'll move towards you, and kill you on contact. Just look around old cylinder crash sites, they don't stray too far from them. (You can find martians outside of machines around too.) When the virus kicks in all non-tripod martians instantly die, and the tripods die soon after thanks to the "sick" stage.