RF stands for radio frequency. It's what cell phones use to transmit their signals. Cell phones can sometimes be used to secretly eavesdrop on conversations when hidden in a room where people are talking. If you would like to know if someone is using a cell phone to listen in on you, one way is to use an RF scanner to detect the transmission.
Purchase a radio frequency scanner. These are available at most online spy and gadget stores (see Resource links). Costs vary, depending on your needs.
Turn on your scanner and place it on a nearby table. Make a call from your cell phone while standing next to the scanner. It will start beeping rapidly. This is the sound you are looking for to see if there is a mobile phone nearby. Turn your cell phone off.
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Move your scanner around the room. You might get a steady slow beep to show that the machine is turned on and operating. However, this doesn't mean that there is a cell phone in the room. If the beep becomes much more rapid, then there is likely a cell phone nearby.
Sweep the room thoroughly by dividing it into quarters and checking each section completely. If you find a transmitting mobile phone, turn it off so it can no longer eavesdrop on your conversations.