So I'm figuring the OP found this either near Green Lake or someplace similar.
Haven't been in Seattle for 2 years now and man oh man am I starting to miss the place.
Back on topic - the OP is just wondering HOW it got there - I would assume he left it right where he found it...I know I would have.
Considering all things Seattle, there are so many reasons it could have wound up where it was found.
It could have been left over from a fraternity/sorority prank played 20+ years ago...
It could have been some old lady's and when she passed her people went through her things and it got tossed in the trash - and somehow either picked up and tossed (what is it with people who rummage through trash?) or fell off the garbage truck... - so those are the 'normal-ish' possibilities...
Then there's the 'only in Seattle' possibilities - remains of a late night outdoor enema session between a nature loving dom and sub (you choose the sex of each and in Seattle, any combo is possible) - I had a friend who is a female dominatrix - she moved from Bellevue to Lynden about a year before I left the area - couldn't figure out why, but maybe them churchified folks up in Lynden enjoy a good whipping?
Oh let's see what other 'only in Seattle' things I could think up...you know, it is odd that it was left behind...had to have been done long before most of Seattle and surrounding area caught the 'save the Earth/recycling/green' bug...who knew I'd miss that?