Posted for general knowledge/awareness:

While anyone with spiritual potential (ie, all series characters) can see a Hole if it is within normal visual line of sight, to locate them at a distance you need one of the devices produced by Urahara, Mayuri, or Szayel.

(Naturally, these devices are prototypes, have flaws, and may not work perfectly.)

Having dispatched his duty to Sixth, bathed and opted for a change of hakama, Renji heads to Eleventh late at night. His intent is to hopefully find Zaraki awake. Or, barring that, to pass the time until Zaraki is. And so he pads through familiar back alleys, eschewing invites to drink, and approaches headquarters.

Summarised here for reference: subject to update if the maintainers have a better idea.

( Further Information on Holes )