I work at medium sized (say 50-100 employees) software industry company for about 2.5 years now, in Germany.
We do not have a strict company hierarchy, so technically everyone can just go to anyone in the company and talk about projects/problems with everyone. We do have regular department meetings and I report our progress to our departments 'representative'.

About a year ago, my department leader told me that a colleague would soon stop working for the company and I shall take over his projects. Everything was clearly communicated, the transition went very smooth and professionally for everyone involved. I quickly learnt about the project very well and was able to make any requested changes easily. I did not leak the classified information that he would stop working for the company - I was just not surprised when it was communicated to everyone.

Now I am 99.9% sure that the same thing his happening again, this time with a different coworker, lets call him Bob. There is project X, which was Bob's project and I joined it about 6 months ago. It was made clear about 2 months ago, that I would now be managing project X, which means keeping track of the progress, but still also contributing code to the project. I realize that I am slowly and unofficially taking over project X, because Bob is leaving or (will be) terminated (soon).

There are several small indications of what is going on. The major indication is that it was recently discussed, that our project is behind schedule, which I could not understand, as progress was great and we clearly were far ahead of the schedule. I was puzzled by this, but then I realized, they must have had additional information, which they were referring to. It was that soon Bob will not be available for the project anymore. They already included Bob leaving the company (or being terminated) in their time schedule for project X. This proved to be true, because I accidentally found a file referring to Project X (publicly accessible within company network), in which I read that it was discussed in a meeting without me, that project X should be finished, while Bob is still with the company. This explains the tighter schedule, of which I had no idea. This file belongs to the person telling me that the project is late.

I have no hard feelings, I am just puzzled about this situation. I understand that it might be a good idea to keep secret the information about people (begin forced to be) leaving the company (for whatever reasons).
However, why would I not be told to take over Bob's duties with a clear deadline, just as it happened before with the other coworker a year ago. For me, each day could be the day at which Bob does not show up to work anymore. How should I manage project X if I am not told about the 'real' deadline and the reasons for it?

I want to clarify this situation and deal with it professionally. Would it be more appropriate to talk about this with my team lead first? Or talk to Bob first (what if he doesn't know / realize what is going on? Talk to the guy who told me project X is behind in schedule and who left this file, indication that Bob soon will be gone? Talk to the CEO first (it would also be easy to talk to him directly)? The CEO must have given the order to everyone, not to talk about it.
Or is the answer: The other people did not tell me for a reason - so do nothing, just go on as if I didn't know it. 

Update
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I rephrased all parts where I told that Bob resigned into something more generic: maybe he resigned, maybe he is being terminated (soon). Everything would make more sense to me, if Bob is not leaving, but is being terminated or that it is planned to terminate him soon.