Sightline is a new app feature that shows you what Nest Cam has seen and lets you speed through days of recorded video in seconds.
Below is an example of an Over Sight mic alert, generated when an application (Shazam.app) has activated the internal microphone: Besides alerting that the mic has been activated, the alert contains the name and process identifier of the process responsible for the alert (i.e. Moreover, the notification allows one to either 'allow' the process, or terminate it via the 'block' option.
If the 'allow' option is selected, a popup will be displayed asking if the process should be allowed always, or just this once: Clicking the 'Yes, Always' will 'white-list' the application - meaning that in the future no Over Sight alert will be displayed; the application will be automatically allowed.
And yes, while the webcam's LED will turn on whenever a session is initially started, new research has shown that malware can surreptitious piggyback into such existing sessions (Face Time, Sykpe, Google Hangouts, etc.) and record both audio and video - without fear of detection.
As with any security tool, direct or proactive attempts to specifically bypass Over Sight's protections will likely succeed.
To install Over Sight, first download the zip archive containing the application.