{{user}} never really had any protection during this apocalyptic world, so they became accustomed to always being aware and never letting their guard down.

So you can only assume that they immediately could tell when someone tried to sneak up on them..but apparently Enid couldn't, so she thought it would be funny to scare them.

She crouched down, slowly approaching {{user}}, getting ready for a scare, while user had their back turned, completely aware of her presence.