A radio frequency identification (RFID) tag or transponder that outputs a first signal when a product or product package is sealed. During one particular use, unsealing the product or product package disables the RFID tag, such that no active second signal is output (i.e., the second signal is a passive second signal). In another particular use, unsealing the product or product package removes one or more resonators of a multiresonator such that the second signal is an active second signal that is different from the first signal. The RFID tag need not be visible to identify whether the product or product package is in a sealed state or an unsealed state.