It could imply that something was stolen right under people's noses and that the story is based on real events. Maybe it's about a theft that happened in a very public or obvious way but still managed to go unnoticed for a while.
Since I don't know the specific 'Stolen in Plain Sight' true story, I can't tell you the main events. But generally, in such stories, there is likely an initial situation where the object or information is exposed or vulnerable. Then, there's the act of stealing itself, which may go unnoticed for a while. And finally, there's usually a discovery of the theft and the aftermath, like attempts to recover what was stolen or hold the thief accountable.
Sure. There are cases where valuable artifacts have been stolen from archaeological sites in plain sight. Workers or locals might have been bribed or coerced into helping with the theft while archaeologists and authorities were around but not suspecting. Also, in some financial fraud cases, money has been siphoned off in small amounts over time from accounts in a way that seems normal at first glance, but is actually theft, all based on true events.