Aliens can learn about earthlings because of cell towers.

The scientists explained that each base station emits a signal with a power of 100-200 watts, and the total energy they radiate into space is second only to military radars. Astronomers have shown that, using large radio telescopes, mobile phone network signals can be picked up from nearby stars, including Alpha Centauri and Barnard's Star. As broadband systems increase in power, the activity of earthlings becomes even more visible. Signals generated artificially will come mainly from densely populated parts of the northern hemisphere.