I don't mean to give the impression that Portland is an unsafe place to park your bike, but one should always be mindful of properly locking their ride up in a safe area and not leaving it unattended for several days. Here's some cautionary tales from around the peninsula.
And FYI, you can register your bike on the Portland Police Department's website so it is on record in the event that the entire bicycle (not just the wheels and such) are stolen.
Last scene is particularly grizzly. The thieves appear to have suffocated the bike with a Hannaford bag before dismembering it.