A method and apparatus to detect a plurality of different security tags are described. A method to detect security tags may include establishing an interrogation zone using at least two signals operating at different frequencies; monitoring the interrogation zone to detect a plurality of security tags, with a first type of security tag responsive to at least a first signal at a first frequency and a second type of security tag responsive to at least a second signal at a second frequency; determining whether to generate an alarm if a security tag is detected by receiving a third signal including a combination of the first and second signals from a first security tag of the first type and filtering the third signal to remove frequency components of the first signal; and generating the alarm in accordance with said determination.