Will you return US$ 2 million check?

There was a news about a guy returning 2 million dollars check today. He was holding the check on for 2 weeks, so he was millionaire for 2 weeks. I guess the check was from Treasury department or something like that. He returned the check in the end. So, if I am that guy, first question is if possible to cash in 2 million dollars check? I won't even question my honesty. If there won't be problem cashing in, I will cash in. But I don't think cashing in 2 million dollars check would be so simple. Bank normally notified and verified with the check writer before letting it pass.

So about being honest. Does it matter to be honest in this case? It's once in a lifetime kind of thing. Why just not enjoy it? I would have framed the check and hang it on the wall if my second thought didn't allow me to cash it in.