Why Couldn't LV get the prophesy for himself?

[mhershey2001]

No: HPFGUIDX 75282

Here is something that has bothered me about the whole "setting Harry 
up to get the prophecy" thing.  I know the implication is that LV 
didn't want to go get the prophecy himself because it would risk 
being seen, so he had to invent a trap to lure Harry there instead.  
However, the entire description of the MoM leading up to the point 
where Harry actually takes the orb off the shelf is one of an utterly 
dark, quiet, closed place in the middle of the night.  I mean, the 
six Hogwarts kids spend what seems to be quite a long time there 
before they get to the orb room, roaming around, making some noise, 
looking at things, etc., and no one comes running - they don't trip 
any sensors, etc.  It doesn't even appear that any Order members are 
guarding the door anymore - when did that stop?  After the attack on 
Arthur?  So my question is, why couldn't LV just have Apparated in 
front of the orb, taken it off the shelf, and then Disapparated?  
After all, didn't he just want to stay hidden until he got the 
prophecy so that he could see what was in it?

Mhershey