Eavesdropping devices can be obvious and not-so-obvious. They can be as small as a grain of rice, which are mostly used by the government, or for corporate espionage. The smaller the device, the more costly it is.
Those devices that are not powered by batteries run on the building’s electrical system. Most of these are found in wall outlets. They can either be transmitting cameras or microphones.
Any good TSCM (technical surveillance countermeasures) person can locate and neutralize all of the above.
One way to see what bugging devices look like is to browse the Internet. Use search terms such as “bugging devices,” “eavesdropping devices,” or “wiretaps.” Mostly, you will see devices that are available from spy shops. Many of the bugs that are sold in spy shops come from overseas because they are illegal in the United States.
On the Sherlock Investigations web site we have a photo of a small transmitter that has a microphone. Our PDF manual, “Bug Off!”, has photos of various eavesdropping devices. It is a free download.