Thanks Helg, that helps a lot.
Actually, I do not know if schools contact recommenders to get more information from them. I can imagine that they would call to verify references, but I don't know about getting additional information. Most schools I have spoked with are aware that doing 4-5 recommendations takes a lot of effort and they try to make it as painless as possible.
Just checked my GSB application...
Recommendations Verified by Office Yes
I will find out if they called, emailed or what.