A mechanic could have been riding along, troubleshooting something in the E&E bay. Some airlines allow this. It can make the passengers nervous, but as long as he has on airline coveralls, they wouldn't be very alarmed.
Would you say that would be a 'usual' practice? In my experience I've had engineers 'ride along' only twice in more than 6 years- once was an overwater sector and an insurer requirement to prevent unsched overnight in an unfavorable port... the other was an empty ferry sector.
Damn straight if someone came up to me saying they wanted in to the E&E bay during flight I'd be calling the captain first.
If they forced their way in I'd be doing a lot more than standing there to watch!
Sadly I'd guess this points to the CC being unable, or as has been put forward, 'someone' in the airline telling them that it was kosher... don't think that is as likely as the first one though...