If after "begin(address)" you set "TWAR = 0xfe", the Arduino will respond as a slave to any I2C address (irrespective of address specified in begin). There is no support in the Wire library however for letting you know what address was actually used. It will be somewhat like a telephone exchange with multilpe inbound numbers. The phone is ringing and you pick up to answer, but will not know the number the other party dialed to reach you.