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Drive slowly. I've seen the icon appear and disappear quickly. It's subtle and I'm usually looking around and not at the dash. Wish it were more visible to peripheral vision.
Also you need to be closer to the parked cars than you might otherwise drive. The sensors need to be able to "see" the other cars and if you're too far away, it won't work.
It seems the sensors are based on space between 2 cars instead of following the lines. It would be good if it can detect real empty spaces. I guess it also doesn't work if there are 2 empty spots?