Is GND actually used in the rest of the schematic, or is it AGND, DGND etc?
"same GND is used in whole sch."
Does the pad show as being connected to the GND net?
"NO, it looks as NC."
Can you reload the part information from the library?
"Yes, but will it help. I need to know the reason why drc is overlooking the error."
What is the state of other pads connected to the GND net?
"yes, other GND pads are connected well."
Is the GND net poured copper? If so what is it doing around this pad?
"yes, pad had poured GND copper, In previous version it is well connected to GND & in revised version it becomes NC. GNd pouring remains there as it is."
Have you got a greater spacing rule set for this pad? preventing copper being poured onto it?
" No specific rules for this pad."
Can you delete it from scm and PCB then add it into scm again and update PCB again.
"Yes, in doing so error gets solved but my concern is why i am not getting DRC error for this unconnected Pad."
Perhaps temporarily try it connected to another net and see the results
"ok, will try."