What is the current return path in this condition
I suspect that the board in your picture is a four layer board, with only the surface layers shown.  There are a number of GND pins which have no apparent connection, so they very probably connect to a ground plane on an inner layer.

However, for simpler circuits it is entirely possible to design a PC Board without any ground or power planes - you use regular surface tracks (but probably wider than signal tracks) for the power and ground connections.