Object not visible in Scene tab until double click it on the Hierarchy.

I found that sometime my game objects are not visible in the Scene tab.
However, when I double-click them in Hierarchy tab, the view in Scene tab zoom out and the objects became visible again. After that it keep visible even if I zoom back in.

How does it work and how do I prevent situation like this to happen?

Thanks.

If you are in 2D, make sure Z coordinates are right. (Probably needs to be 0)