The two words which you can use here are 'undisclosed' or 'confidential', but the meaning is different. 'Undisclosed' means that nobody has made the results public, but we don't know why. Maybe the person/organisation who has the results has their own reasons for not publishing them. 'Undisclosed' means that the report has been kept secret, but it may or may not be confidential. 'Confidential' is a category of information. Imagine a big red stamp on a file, saying 'Confidential!'. It means that someone has classified this report as not for public viewing. Of course, not all confidential reports remain undisclosed. If confidential information gets 'leaked', it is revealed to the public. As for your 'lid' metaphor - no, it's not quite right. We normally say 'Keep a lid on it' or 'Keep the lid on it'. It's an image of a simmering pan of on the stove. Remember - with idioms you have to say them exactly right, or not at all.

I would say confidential. But the other expressions mean similar things so , except for "covered" you could also use them. "To keep under the lid" also means to keep something secret, not to reveal it.