Master Mode: High-Speed Token

When enabling the high-speed master mode of Tracii XL I can see transfers to an I2C slave with address 0x04. As there is no slave with such address on my I2C bus, these messages get no acknowledge. Who sends these messages to the I2C bus and why?
The I2C protcol specifies a high-speed token, which has to be sent to the I2C bus before going into high-speed mode. This token looks like “START, 0x08, NACK”. This token is sent to the I2C bus with a SCL clock up to 400 kHz. After this token the real high-speed transfer starts with a “RESTART”.