Why do I have to complete a Captcha?
Completing captcha proves you are a human and gives you temporary access to the web property.
Why am I not seeing any Captcha?
Probably because access to the site permanently blocked from your network.
What can I do to prevent this in the future?
If you are on a personal network, you can run an anti-virus scan on your device to make sure it is not infected with malware.
If you are at an office or shared network, you can ask the network administrator to run a scan across the network looking for misconfigured or infected devices.