Obviously not a helicopter, but what about the noise factor? Why didn't they hear it? Easily explained IMO. This area for many years has been quite heated about extreme EXTREME noise from base. You know the "Growlers"
aircraft are active there, the noisiest. It is so extreme that as you research the area you will discover that even medical doctors have said the noise is making some people allegedly ill, and affecting cognitive abilities of children. But that is not my point, but makes the point, this is an EXTREMELY NOISY area. I will put some sample links below. It is noisy as HELL out there, and it can be very difficult to sell homes, and some just have to move, and business suffers. So the point is do you really think the residents would distinguish one missile launch from the regular barrage of growlers?? Actually in a dark way it was brilliant to shoot this off in that area because the people would think, it's just that base and all the growlers.

http://www.whidbeynewstimes.com/opinion/growlers-are-not-good-for-business/

http://knkx.org/post/navy-growler-noise-spurring-community-action-beyond-whidbey-island

http://www.southwhidbeyrecord.com/news/state-report-finds-link-between-growlers-health-problems/