I'm looking for a way to hide and encrypt something on a flash drive. Someone who has expert knowledge is going to explain in simple terms how it was done and how they broke the encryption. (I know writing it is my responsibility! I just don't know where to start here.) Ideally it should be a way it would take a person with pretty advanced knowledge to figure out.
To explain more: The drive looks like it contains something relatively innocuous like pictures or text (I don't care which) but really it contains someone's top secret text and pictures. I'm very flexible as to how this happens, but I have no idea how it could happen myself. (I do remember a Law and Order SVU episode where someone encoded some text as a tiny part of a picture, which is the general sort of thing I'm looking for. But it's not an SVU fanfic so I'm nervous about using their material unless this a real thing, plus I can't remember how it was explained on the show.)
One of the problems is that someone who isn't as much of an expert already failed to notice the secret files because they weren't looking very hard. So ideally I don't want something that would be totally obvious to any person with basic computer security knowledge. (Though I guess I could make the first person totally incompetent if all else fails...)
I'm not actually asking anyone to tell me the details of how to do practically do this! I just want to be able to say something slightly better than "He sat at the computer for a long time and then suddenly said 'Hey, there's something else on here!'"
I really don't know what to Google. I tried "hiding files on a computer" but it got me ordinary Windows stuff and commercial programs that don't sound like quite what I'm looking for. Similar results with "file encryption."
Thanks in advance for any help.