What is that building in Suvarnabhumi Airport?
On page 5 of current airport chart states it is Transfer Baggage Terminal

It was officially opened on 1 November 2014 and reduce baggage transfer time from 75 minutes to 60 minutes. It was constructed above South Airside Tunnel between gate C, D and E and has usable space of about 12,550 sq.m.

Source (Sorry source is in Thai. My own translation.)
Another source in English. Similar to my original source.