In a research paper called "The population of natural Earth satellites", astronomers say that Earth must have a second moon at any given time. They have calculated the population of "irregular natural satellites that are temporarily captured" by Earth.
These secret moons come and go quietly and without notice, but at least once we detected one. In fact, their calculations were confirmed by this observation, a mysterious titanium white object that was discovered rotating around Earth in 2006 by the Catalina Sky Survey in Arizona.
"Our results are consistent with the single known natural [temporarily-captured orbiter] 2006 RH120, a few-meter diameter object that was captured for about a year starting in June 2006."
That object was actually a small asteroid captured by Earth's gravitational field, and it rotated as a second moon until June 2007, when it left its orbit. This study demonstrates that, even while they are not detected, these little moons come and go often, staying around for about then months, take about three spins around the planet and then wave goodbye.
Imagine being able to detect one of these tiny moons and send a few astronauts to capture it instead of getting them to a distant one. The information that we can obtain could be phenomenal. And without having to spend a lot of money.