Understanding data holes in a Intel HEX file
Intel Hex specifies the bytes to be programmed and their new values. All other bytes are keep their previous state. If you are programming a blank part, that previous state is its unprogrammed value.
Intel Hex is targeted at programmable (non-volatile) memories. Nearly all programmable memories used over the last decades are EPROM variants, such as Flash. Their unprogrammed value is almost always 0xFF. Note that the datasheet must always be used to obtain the actual unprogrammed value for your part. As with any characteristic, it should not be assumed.