MacBook takes screenshots without user input Yesterday my desktop MacBook with Yosemite took a screenshot although I did not use that facility. It made the sound and created a file that I threw away fearing it was a virus. The doc created was not in fact a photo from the camera or the screen.

Today it happened again: the same sound of screen shot, but this time there is no file to be found in the computer.

Can this be a virus or an intruder?
According to Trend MICRO (September 12, 2016) the symptoms you describe do confirm that this could be a virus or intruder:

Infection Channel: Downloaded from the Internet, Dropped by other malware

This backdoor enables the attacker to steal a range of information, including screenshots, audio, and keylogs. It can also execute commands and communicate using encrypted data.