Around now (7 months) is the time when the child should be learning "Object Permanence"- the fact that things still exist after they leave his line of sight. The classic description of this is something like:
"If you get the baby interested in a toy, then cover the toy in a cloth, the baby will remove the cloth to find the toy".
But I don't know how to test this in practice. The problem is that I can get him interested in a toy, but then the act of covering it is moving a big white moving cloth across his field of vision, which gets him interested in the cloth. So he removes it, revealing the toy. But I'm pretty sure he's doing it because he finds the cloth interesting, not because he's removing an obstacle in the way of the toy he'd looking for.
And now that I'm dealing with it in practice, I can't figure out a way to hide something he's staring at without giving him a distraction to go after instead. One thing I tried doing was dropping a ball into an opaque box. He gets interested in the box, but again, that could be because he just saw lots of color and movement in the vicinity of the box.
So yeah, I don't know..