According to a police official, the robbery may have taken place during the weekend holiday but came to light when the branch opened on Monday morning.
Investigators said the tunnel was dug from outside to the bank's store room which houses private lockers.
But the robbers could not break open the main safe vault housing the bank's cash reserves.
The robbery - reminiscent of the 2008 British-Hollywood flick "The Bank Job" - created a sensation.
A police official said it could be the work of expert criminals.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)