Grad students/postdocs taking ideas with them, what's normal?
I think the main thing you should keep in mind is, did it seem like people in your old lab/group will be or would like to be working this?  You don't want to end up "stealing" someone's project, or competing with them.

There's a difference between an idea you've come up with completely on your own, and something you've come up with together with other people.  For ideas of the latter type, it's reasonable to suspect that your former colleagues may still be thinking about this.  In addition, if they contributed significantly to the ideas, they should have a chance to be included in the project.  If how the other parties feel about working on this idea is not clear from your prior interactions, just talk to them about it.