Research Electronics International, a leading manufacturer of counter-surveillance equipment, and having one of the foremost training facilities for Technical Surveillance Counter-Measures, says:
"A physical search is the root of all counter-surveillance work. It also overlaps other procedures. Be thorough and look closely for anything suspicious. Your physical search may be the only method to uncover wired microphones, fiber optic microphones, passive resonators, inactive remote controlled 'dormant' devices, or devices which are not generally detectable with counter-surveillance equipment."
This is one of the reasons why sweeps take so long. Of course, you have to know what you're looking for. People have showed all kinds of things to me on jobs, thinking they were eavesdropping equipment. New eavesdropping equipment is coming out all the time. A TSCM professional has to keep apprised on the new equipment.
One thing that is important to know is that every device must be powered, either by batteries, or the building current. I've seen "bugs" (transmitters) powered by hearing-aid batteries. They're very small!
I've also seen bugs powered by the buildings current. They're typically placed in wall outlets.
Yes, electronic equipment is necessary for a bug sweep, but you can't overlook a physical sweep too.