Potential evidence of a bug or listening device include strange noises on the telephone, changes in the volume during conversation, or person not connected with a situation knowing more than they should. These are the most common signs that eavesdroppers may be listening to private conversations, or have been able to get confidential information that was not intended for them. Additionally, be wary of unusual or unexpected gifts that are meant to be kept in an office, or other normally private area, particularly if they are from an unknown sender.
Tapping a phone line is illegal, but it is still sometimes done for the sake of eavesdropping. The phone may make a clicking sound when it is picked up or the person on the other line may sound very far away when they are being spoken to. Law enforcement should be contacted immediately when there is suspicion that a phone line is tapped.
Another way that eavesdroppers choose to listen into conversations is by bugging something that will sit in the office. They may send a framed picture, a reusable vase or a small sculpture to the office that they wish to bug. The object is often equipped with small wiring connected to audio devices in order to pick up conversations and send them back to the person who commissioned the gift.