None of those things, perhaps with the exception of new technology, but that will most likely be military technology, and if there were any commercial opportunities, it would not remain hidden.
You have to consider motive. What motive does the government have to hide alien encounters?
If they're hiding anything, it's not politicians, but policy advisors. Governments often look for the evidence that will best support their predetermined policies rather than the best evidence on which to ground their yet-to-be determined policies.
EDIT: Example: The UK government's instructions to its Joint Intelligence Committee (JIC,) in making a case for invasion in Iraq (similar things happened in Australia and America), requested a case “as strong as the evidence properly permitted”. This amounts to a request to slant the evidence, and thus to depart from a strictly objective standpoint. A truly objective account would surely have asked for a case “as strong or as weak as the evidence properly permitted".
Further, the concept of 'plausible deniability' is relevant here. By this concept, political leaders are deliberately kept in the dark in order to be able to deny personal knowledge and therefore responsibility for disreputable decisions. Here, Iraqi WMDs come to mind.
It's not that they're hiding anything, but are operating with 'impaired' or incomplete knowledge, without awareness.
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