Apologies. The rules are a little ambiguous on this. if you're hiding and someone wants to spot you, you make opposed 4D checks.
Shrinking adds to your check if you're the one hiding. Something like Enhanced Senses would add to someone's check trying to spot a model in hiding.
If someone's not actively hiding, just have anyone looking for them make a TN2 check. For a Shrinking model, you can increase this to TN3.
Hope this helps!