lots of stuff to unpack here.
1). First off per RAW i think Xenon has it right; the rules do not even mention anything about making something other than a willing person invisible.
2). IMHO I do think that some kind of resistance test by an unwilling person (or the tech resistance table for an object) makes complete sense. It's just not spelled out in the Illusion spells section, hence Xenon likely has the RAW of it.
3). If you accept the logic of item 2 above then imho it also makes sense to take the net hits above the resistance check as the net successes for someone to pierce the invisibility. That way it's only ever one roll for the caster (instead of "one roll to effect the unwilling thing" and "another roll to gauge how invisible it is").
4). This does open up another interesting question. Which is "what is a target"? I mean if you put 6 grenades in a bag and you cast invisibility on the bag will the grenades inside still be invisible? If you cast invisibility on a van full of runners will the van become invisible with all the runners still visible inside floating in the "air"?
5). Given the problems that item 4 opens up then it might be better/ easier to rule that invisibility only affects something with an aura (i.e. a living being) and instead require a different spell (such as Trid Phantasm) to make a bag of grenades or van full of runners "invisible" then have the mage concentrate on the illusion to move it with the item/ items in question. This would infer a clarification on invisibility that it only works on a single living being per casting.
what do you folks think?
Whether or not the target of the spell is supposed to resist being affected by it, I just want to make sure it is clear that Object Resistance is an opposed dice pool, not a threshold. Extremely advanced objects roll 15+ dice to resist, not set a threshold of 15...
But it's less clear what impact the OR has, if any, on the subsequent checks observers have to see the invisible drone.
Example: Mage casts imp invis on the drone, as an advanced/technical object it resists the magic. Lets say mage scores 5 hits, OR scores 4 hits. Ok, the Drone is invisible since there's at least 1 net hit.
But when the drone tries to not be seen by observers, does the observer have a threshold set by the mages total hits or net hits vs the OR? I'm not sure that's clear. And it's in this case a big deal... 5 hits vs 1 hit to resist the invis... I'd personally lean towards the net hits after OR (so 1 hit is all that's needed to resist the invis in this example) but I can see where others would argue use the total/original hits (5 in this example)