A small hidden microphone and a radio transmitter are generally known as:

A small hidden microphone combined with a radio transmitter is commonly referred to as a "bug." This term specifically denotes devices designed for covert surveillance, allowing someone to listen in on conversations without being detected. Bugs are typically used in espionage or security contexts to gather information discreetly.

In contrast, a wiretap refers to the interception of telephone or internet communications, usually involving a legal process. A beeper is a term more associated with paging devices that alert the user to incoming messages or calls rather than recording audio. Electronic surveillance encompasses a broader category that includes various methods of monitoring, such as cameras and audio recording devices, but it does not specifically pinpoint the combination of a hidden microphone and radio transmitter, which is why "bug" is the most accurate answer in this context.