The good news is I know where it is, the people at the restaurant know that I know, and that umbrella is in safe keeping with my name on it (literally; there's a note).
Okay, backing up: I worked until about 5 o'clock, then walked off Pill Hill, counting my steps on certain stretches of road (using my mind, not my currently-wonky pedometer) and marking them down in a little booklet. At my stride, by the way, a quarter-mile is about 500 steps. (And by the way, the farthest I can walk in my studio apartment, from the corner of the bathroom to the corner of the kitchen, is 20 steps. NOT a big place.) I reached the Streetcar line at Portland State University, then walked along it until two streetcars, the first almost full, the second almost empty, rolled up.
I hopped on the second train and rode it to NW 21st (a.k.a. Trendy-First) Ave., then walked down to Cinema 21 to see if I could get advance tickets for Spike & Mike's Sick & Twisted Animation Festival tomorrow night. I couldn't, but I can anytime tomorrow afternoon. I then stopped for dinner (pizza and a Caesar salad) at Pizza Schmizza.
Which is where my umbrella now waits, because I was a dumbass. It didn't even dawn on me I was umbrella-less until 30 blocks and two buses later.
I'll spare you the rest of my thought process: soon as I got home, I dug out the phone book and called, and was much relieved that my really, really nice and big umbrella is in safe keeping at a good place. And I was going back to that neighborhood tomorrow anyway, so...it all works out!