Bullet point thoughts
1.Opportunistic bugs and not permanent ones.Fixed with easy to carry adhesives whenever a sensitive discussion was held with the complicity of an insider. Removed thereafter.Permanent bugs will not be fixed to walls. Will be fixed inside walls during construction or fitted clandestinely inside objects like ash trays, flower vases, landline telephones, table-lamps etc.
2. Not sophisticated transmitters. Sophisticated transmitters fixed on the wall would have interfered with mobile telephone functioning and immediately created suspicion.
3.Objective---Not interception of telephone calls, but snooping on discussions of the FM with aides, confidantes etc.
4. FM asked the CBDT to do a sweep because he probably did not trust the IB. He could have suspected that IB was snooping on his conversations/discussions.
5. If his suspicion was correct, the IB would not have done it on its own. Must have been on orders. Who gave the order?
6.If not the IB (unlikely), could be only corporate houses or suspected parties in corruption cases through private detective agencies.
7. Too late to find the truth or reconstruct. One can only speculate.
May develop this into an article later in the day. No time now as I have an important engagement whole of today.