As a reminder, if a human explorer had seen this, we'd already know much, much more about it. They'd naturally take a few steps over for a closer look as soon as they saw it.
Robots are a poor substitute for humans when it comes to planetary exploration.
A human explorer probably wouldn't have seen it though; it's really tiny. The reason this was seen at all is that the high resolution images are being looked over by a large number of people on Earth. NASA themselves didn't even spot it.
(High resolution imagery + many many human eyes) > Human eyes
Never mind that Curiosity has now been operating for 6 months without food, water, or sleep, and is expected to continue doing so for years.
It's shiny, the odds are very high that a human would indeed have seen it. For one thing, a human's eyes are constantly processing the world as they move through it. Curiosity's eyes are not.
What Curiosity has done in 6 months could have been done in a couple of weeks by a human, sleep time included. Robots are vastly inferior to humans for this work, and will remain so for some time to come.
Robots are a poor substitute for humans when it comes to planetary exploration.