I have a very valuable co-worker (I'll call him Joe). He is my senior at the company by a couple of years. 

I am afraid he is thinking about leaving our current company. Without being too specific, Joe had an expected bonus which was not guaranteed, but which was deserved (from my point of view). He did not get it. From my perspective the main reason he didn't get it was bad planning on the company's part.

In the past few weeks Joe has taken a few unplanned half days off. As a friend I asked him what he was up to (I assumed he was just doing something fun). Joe quickly replied "Nothing". I suspect he was interviewing. Before this point I already had concerns about Joe leaving. 

My question is if/how to mention my concerns about Joe leaving to management (I am not a manager and somewhat new to the company).

I already mentioned it to my direct manager (Bob) in a one-on-one meeting. I stated that this is pure speculation from my part. He wasn't aware that Joe would leave over the bonus. But he mentioned he appreciated me bringing it up. I am just concerned that information is not getting up the chain. The particular manager (George) in charge of giving this bonus is a couple levels up. I think Joe would hesitate approaching him directly because doing so may be putting a "target on his back".

If Joe does leave, the damage done to the company (I estimate tens of thousands of dollars) will be far more than the cost of the bonus (a few thousand). I work with him closely and know his performance is top notch. He is regarded by my peers as a great asset to the team. He helps me out on a daily basis, and from a selfish point of view, I would hate to see him leave. I think he wants to stay, but has been slighted by the company a couple times already before, so he isn't afraid to leave. I think if the company gave him the bonus, it would go a long way to keep him. 

My question is: **Should I raise my concerns that Joe could be leaving to my managers?** Should I go to Bob again? Should I go to George? Should I talk to Joe directly?

UPDATE: I apologize for updating this so late. A couple of key points that may affect the answer to this question

1. This "bonus" was in the form of an initiative that all employees were made aware of. Everyone knew the possible bonus would either be a smaller X or larger Y depending on performance. In short, it was known nobody got Y, so it seems Y was not possible all along.
2. The person in charge of this initiative(George) was a few levels up. He ultimately had to decide who would get the bonus. Bob, Joe's manager(and my manager) is of the view point that that Joe did deserve the bonus. But has told me he feels it's between Joe and George.
3. The culture and workplace is great. Bob is a great manager. Joe is a great co-worker. Part of my motivation for wanting Joe to stay is not to have change.
4. While Joe does help me, I would be fine without him. He makes my life easier and is a good guy to have around. That's why I want him to stay.
5. I wish to convey the manner I brought this up. I did not do it to "tattle". I did it this manner: "Hey, you do realize that Joe being shortchanged like that could lead to him leaving, right? Joe does a lot for us, it would be shame to lose him over Y dollars". I didn't mention it to anyone but my boss. I am not spreading rumors around.
6. Joe is so important to the company, even if the company thought he was leaving, they wouldn't punish him in any way. On the contrary, they would try to figure out how to get him to stay(reasonably). Joe wants to stay, but he feels a little short-changed. Just my point of view, but Joe's career is not at stake here.

Again this is from my point of view, so take it for what it's worth. But I will say making assumptions about my culture, my performance that are untrue are unhelpful and I won't make an answer as correct that does such.