HMS Bounty and park-goers in The Voyage Home

In The Voyage Home, why didn't park-goers bump into the cloaked Bird-of-Prey (also known as the "HMS Bounty") more often?

It seemed to be a fairly active park in an urban area (San Francisco). Shouldn't more joggers be colliding face-first into the landing struts? You might imagine that frisbees and baseballs would have been bouncing off of the hull. What about birds dropping out of the sky after banging their noggins on the invisible obstacle?

You would also think that a few incidents of walking into an invisible object would cause a stir of some kind.