A few years ago I stayed at the Marriott Boston Copley Place and returned to the hotel one afternoon to find a picket line (with incredibly loud whistles blowing) on the sidewalk in front of the hotel entrance. The employees themselves were not on strike, but apparently the property owner was using non-unionized labor to do the hotel renovation, and a local union was picketing.

Not that this would have discouraged me from staying at the property (thankfully there was no sound in my room), but as another poster wrote, the guests certainly did not have any advance notice about this.