Snape as an Occlumens

[Penny Brooks]

No: HPFGUIDX 106193

I have a question, and I've done some searching of files to see if 
it's been discussed previously--I haven't found anything, so 
hopefully it hasn't.  Here goes:

In OooP, we learn that Snape is an occlumens, and presumably a 
legilimens, when he has to teach Harry occlumency.  What I'm 
wondering is:  why hasn't Snape used this skill before?  There are 
many situations in the previous books where Snape has accused Harry 
of doing things he did/didn't do, and if he were penetrate minds, as 
he does in the occlumency lessons, why didn't he do so at those times 
to learn the truth?  An example...in CoS, Hermione steals ingredients 
from the potions cupboard, but Snape accuses Harry...all he'd have to 
do is a little mind-reading, and he'd know it wasn't him.  It seems 
that Snape's animosity for Harry would give him a propensity to do a 
little mind-reading on Harry.