Have you, by any chance, built an app that faces the web and written it inside client (browser) JS code? Or have you made an app where you put the API key in? Or something along these lines?
Is that Wordpress plugin written in PHP or JS? I’m not proficient in WP, but if it’s JS I’m almost sure it gets exposed to the client, regardless if you block folders and whatnot. As soon as you expose anything client-side, bad guys can read the JS and extract keys and other sensitive information.
If you want, you can DM me your page and I’ll take a look to see if it exposes it.