Interesting! I definitely tried to observe that dual targeting behavior, but the behavior was similar enough to when people were targeted alone, I didn't notice. Something that might make this more of a reasonable discovery is if I could tell myself a story about what is happening.
As it is, the drones seem to be able to independently target multiple targets independently, so it's unclear why a new target would require the initial rectangle to get bigger.. But if there was a way to communicate that the drone system has some kind of "switching-cost" when there is a second target, then that could be effective in allowing the user to intuit what's going on.
As an example solution, perhaps the red box pauses for a fraction of a second and changes to a more transparent color when a secondary target appears, giving the appearance of being "inactive" while new targeting is happening, and then resuming the shrinking. This could be intuited as extra processing time required. It might even work without the transparency effect.. Anyways, just some thoughts, nice work!
I just assumed there was a single drone and, as you've explained, extra target would force it to perform cost analysis. Please note, that this is a 1KB game, and I really had no more space to fit in any kind of extra effect to hint the player. Even a single sound. Well, maybe if I have refactored it more I could spare these couple of bytes...