The phrase he's in the walls circulated online as a standalone meme line detached from a single confirmed source, shared mainly as a short audio clip and caption. It spread through meme pages, gaming edits and pet videos where the joke is exaggerated paranoia about something hidden just out of sight. Creators reuse it because the wording is instantly clear even without context, and the looping panic works for cats, horror games and conspiracy jokes alike. Its popularity comes from that mix of creepy premise and silly overreaction.