You could put this captcha in a location that wouldn't be very visible for a human, but if the LLM is looking at the HTML he would find this form.
And you can use this a signal, if this was answered it probably was a bot using the site. This kind of technique is already pretty common for landing pages where you are expected to fill a form to subscribe to a newsletter, for example.
alt Hacker News
You could put this captcha in a location that wouldn't be very visible for a human, but if the LLM is looking at the HTML he would find this form.
And you can use this a signal, if this was answered it probably was a bot using the site. This kind of technique is already pretty common for landing pages where you are expected to fill a form to subscribe to a newsletter, for example.