Process IDs in a multi-namespace world

A global pid is context information that probably shouldn't be shared across "context boxes".
You could possibly figure out what pid a "targeted" process is by knowing about when it 
started and what processes started before and after. This would be useful in an attack 
breaking confinement. I know this is all theoretical, and may not even be possible, but if 
you can design it out you don't have to seriously think about it. 

Why should processes in one box have any knowledge of the processes running in another 
that don't explicitly announce themselves? If you are relying on this then you are really 
in the same context (process namespace) anyway. It doesn't matter that the containers 
are visible or not, this is one of the points in having containers with namespaces in 
the first place, I would think.