Click on the Detailed Analysis icon for the specified date and time. After the section "Check Details", there is an HTTP error code section. Most likely you will see this type of response code:
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
An HTTP 301 in this situation means that your monitor went to https://www.myhost.com, got redirected to another URL which did not exist anymore (https://www.myhost.com/US/Home), and then was redirected back to https://www.myhost.com.
Browsers won't show this issue as it moves too fast for our eyes to capture, however will be displayed in ASM, which is why the site appeared to be up that entire time.