What do the Borg do when they aren't assimilating?

Question

When we were first introduced to the Borg in Q-Who, the cube was travelling somewhere when it notices Enterprise and decides to poke it.

Worf: Captain. The cube has already changed course to intercept us.

I can already anticipate the comment that the cube may have been on its way to assimilate someone else, so I'll provide another example where the Borg drop everything to chase down the Federation. Via Voyager Dark Frontier:

BORG [OC]: A vessel has been detected. Unimatrix four two four grid one one six. Activate. Alter course to intercept.

Emphasis mine.

Also, Hugh was found in an area that didn't really have much in it, per I, Borg:

Captain's log, stardate 45854.2. The Enterprise is charting six star systems that make up the Argolis cluster, an area being considered for colonisation.

Emphasis mine again. It doesn't look like they ever asked Hugh what his ship was doing there, or why it crashed. It seems like there's nothing around to assimilate.

So, When they're not assimilating people, what do the Borg do? It certainly seems like they engage in other tasks.

Answer 1

Apparently, in their free time the Borg patrol their space, as seen in the episode "Unimatrix Zero, Part II":

AXUM: It turns out I'm on a scout ship patrolling the border of fluidic space. On the other side of the galaxy.

and also do research, as seen in "The Omega Directive":

SEVEN: This is a harmonic resonance chamber. The Borg designed it to contain and stabilise Omega.

JANEWAY: Here's to Borg ingenuity. This is excellent work, Seven. We may need this.

They also spend time retrieving their damaged technology, as mentioned in the Voyager episode "Unity":

TUVOK: Perhaps. However, we should avoid complacency. The Borg are known to retrieve their damaged technology. It is quite possible we will encounter additional Borg vessels which are fully manned and powered.

Answer 2

In answering this question you have to consider that the Borg are not individual entities, all Borg are connected into a collective and have distinct roles and tasks they are assigned to do. You also have to consider what there ultimate goal is. As viewers the only thing we absolutely know is that they have a single minded goal to achieve "absolute perfection" If you look online there are a number of theories about this, none of which have been confirmed as canon but the most popular one is that the Borg will not be happy until they have assimilated everything that they see as worthy of assimilation in the galaxy (and in scorpian we see that desire take them to fluidic space). Or destroyed anything that is seen as inferior. The reason for this is that the Borg are incapable of true innovative thought, they rely on the knowledge that they assimilate from other species and then apply that knowledge to their own technologies rather then engaging in indepth research on blue sky ideas. Even the Harmonic Resonance chamber was created after the correct technologies where assimilated in and then applied.

As the Borg all serve the collective, and as the collective believes in complete efficiency then you can assume that all borg, no matter what they are doing, are serving this greater goal. Yes they have roles outsode of just assimilation. as an example

There are Borg assigned to protect and patrol the edges of Borg Space hunting down anyone who may attempt to invade, attack or simply wander in.

There are Borg who will be searching for and collecting any cubes or drones that have been identified as having been damaged, destroyed, captured or lost.

There are Borg who are working soley on building the Borg infrastructure, manufacturing the borg ships, transwarp conduits, combing through the knowledge and technology assimilated to identify what is worthy of adding to the collective and what knowledge is not. Attempting to understand technology that they cannot investigate.

There are Borg who's role is to act as scouts, searching for and identifying the next targets for assimilation.

And there are Borg who are being sent on missions to assimilate and learn, such as cubes sent to the Federation.

But there is no concept of Borg free time, if a Drone is not required it sleeps going into hibernation until needed. If a Cube is not required it goes dark drifting in space until required.

So what do the Borg do when not assimilating, lots, but all is aimed towards that singular goal. The assimilation or destruction of the species of the universe the collective and no single borg would ever do anything that did not directly or indirectly help serve this goal.

Answer 3

They steal stuff.

As discussed in this answer, when we first hear of the Borg, they are destroying outposts apparently by just taking them:

DATA: Captain, there is nothing left of Outpost Delta Zero Five.
LAFORGE: Must have been one hell of an explosion.
DATA: Sensors indicate no evidence of conventional attack.
PICARD: Can you determine what happened?
WORF: The outpost was not just destroyed, it's as though some great force just scooped it off the face of the planet.

(TNG: The Neutral Zone)

They do the same in "Q-Who". While the outposts have been manned, we never hear from the people there again. The exact same thing happens to the Romulan outposts in the Neutral Zone, but a scene in Picard indicates that no Romulan was actually assimilated: In "The end is the beginning", Hugh describes a few Romulan Borg as the only ones ever assimilated.

So either this is a plot hole or the Borg actually stole their technology without assimilating anyone.