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Since it only shows up on the bike activity, I'm assuming it's people parking in the structure and the GPS losing accuracy inside.

Interestingly, the same patterns happen in Vancouver when downtown near all the highrises: http://labs.strava.com/heatmap/#16/-123.11986/49.28436/gray/...

The noise may well be indirectly tracking building height. ;)

They might be riding it up and down some too, the connections to the blob don't have much intensity.