During the "Drone" episode, the mobile emitter's technology is used to create a much more advanced drone than what the Borg are currently capable of producing, and they detect this drone and focus extra attention towards obtaining it - Why don't they focus the same amount of attention towards the device that created it?
Before the Drone episode, a plausible explanation is that they simply haven't realized the potential of the device, however this seems unlikely given how adept the Borg are at placing different values on various assimilation targets, combined with the fact that they have scanned the entire ship multiple times.
After the "Drone" episode, do they believe the device is destroyed or unusable somehow? without even bothering to scan again for it?
Another possibility is that perhaps they are simply incapable of assimilating technology that advanced, however that doesn't mean that they can't obtain it and store it for study - or is it no longer worth it if they do that?
edit: It seems I need to clarify the value of this device to the Borg - when a drone was created using this technology:
As a combined product of 29th century and Borg technology, the new drone possessed several unique features and abilities. These included internal transporter nodes, body armor composed of the same poly-deutonic alloy as used in the mobile emitter, and a multi-spatial personal force field. In essence, it was a 29th century drone, immensely more powerful than 24th century Borg.
Basically it's not just about the holo technology, which may or may not have benefits to the Borg.