Not confirmed of course..but i m pretty sure some VERY SPECIFIC CHARS...NOT ONLY ARE CAPABLE OF DETECT, AND LOCATE HAKI,, if NOT Know the true nature of it.... in a similar way at how VOA works.. being the user able to read, the NATURE or Intention for example in a Phoneglyph... similar at how Roger did..
I got the feeling that just by the NATURE of the Haki.. not just cause Imu´s is crazy strong,, chars like Roger get a glimpse of HOW FREAKING EVIL he is...
and thats where stuff like "the world" come from,, even when i dont know exactly what he tried to mean with that,,but it is the same Xebec detected,,
and the reason why BB pirates want "the world" in a reference to IMU´S and not in a sense of "Conquering the world"
and something similar happened to luffy in the very very last chapter he was showcased in elbaph,, when he not only detected GABAN was falling...
if not the NATURE and INTENTION of Imu´s haki...thats why he tell Gaban..that, he doesnt need to explain anything,, just feeling it alone,,,he knows he needs to get rid of it at ANY COST..