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 begins immediately.

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