My mom was a night guard, then a janitor for many years. I tagged along to some places she worked. I got to play and hang out after hours in the interesting buildings. But yeah, we were "invisible" so to speak. People were often impolite and rude to us because they saw us as beneath them. I always make a point smile and greet the janitors and other service workers I encounter, partly because I know what it feels like to be "invisible".