The first of these videos I saw was the WDR Radio Orchestra showing up seemingly out of nowhere in the Cologne Wallrafplatz in October to play the Star Wars theme. Cute and fun. (About two minutes.)
The next I saw was from the Copenhagen Metro in April, where the Copenhagen Phil played a piece from Peer Gynt on a subway car. Lovely and nicely done, though it kinda looks like half the people on that car are involved with the orchestra. (Also about two minutes.)
Now Jay points to a video of the Youth Symphony Orchestra in San José, Costa Rica playing the “Waltz of the Flowers” from The Nutcracker. This one is five minutes long, but I think it's my favorite of the three (even though I'm not a Nutcracker fan); among other things, I love the sudden flight of birds at around 0:40, and the little kids running through things, and more generally I think the dance aspect makes the musical-flash-mob idea even cooler.
I guess this must now be a Thing that orchestras do—show up out of nowhere, perform, and depart as swiftly as they came. I've seen similar kinds of things done by Improv Everywhere and Prangstgrüp and others, but seeing an orchestra do it is new to me. I like it.