Two-way radio squelch tones
The most probable signal to be sent out at the start of a Police/EMT transmission is a Digital Selective Calling identifier and/or selector. This is a series of audio tones which digitally identify the transmitter and select the receiver or group of receivers which are to receive the call.

Your best bet for finding information on this is to look for Digital Selective calling or DSC in the owners manual. Also this http://www.w2sjw.com/radio_sounds.html has a collection of examples.

On less sophisticated equipment a beep is sometimes added when the unit stops transmission. This is referred to as a roger beep or jokingly as an idiot beep and is used to indicate end of transmission without using good procedure.