Martian Probe

The Martian Probe is a rare Hardmode, post-Golem flying enemy that can spawn anywhere on the surface. It flies through blocks. The Martian Probe initiates the Martian Madness event if it detects a player and is then allowed to escape off-screen.

A Martian Probe flies around, attempting to detect players; if successful, it turns red and attempts to fly away. Destroying the Probe prevents the event from occurring, but if it escapes, the Martian Madness event will begin immediately.

Additionally, it is believed that a Martian Probe spawns for every Golem defeated per world.