Hey there. Quit. You're thinking realistically. You're thinking that asteroid belts are really non-dense, and that any ship can just come in to a system, do a visual scan of the place, and have a good idea of anything big or hot or whatever, and then leave... and that stealth doesn't work in space.
This is a realistic view of things.
It also isn't how this setting works. Have you played Wing Commander? Or most any of the 'ships in space' science fiction games? Homeword, Nexus, Privateer, Freelancer, Freespace, various Star Wars games, etc.? Where Asteroid Belts are really really dense, and huuuuuge pirate bases can most DEFINITELY hide in asteroid belts (as long as they are built out of the rocks themselves? And that space sensors are REALLY short range and tend to kind of suck? And you have to actually send craft into a pirate owned asteroid field, and they have to get REALLY CLOSE to the base, scan it, and have them get out alive, and then back to a place where they can jump out for them to successfully have intel on the base? Yea, it's like that.
So, yes, it's plausible to consider that everyone in the setting has sensors worse than late 20th century telescopes plugged into late 20th century computers, doing visual and infrared scans of the area around them... however you want to make that out, I suppose. Anyone want to take a shot at why this is?