How do the aliens track and decipher these "trails"?
One obvious part is cellphones. People who go hiking in wilderness and carry cellphones carry radio transmitters, which tell them apart from other big mammals. And the cellphone bands pass through ionosphere, so satellites could listen to cellphone signals. But how good, really, are spy satellites at wiretapping cellphones?
One part: can they? When they detect someone hiking away from settlements, they can launch UFO raid, so that if air traffic control spots the incoming UFO and scrambles fighter jets and helicopters, the UFO is out before the jets approach the vicinity. But this is a high profile, detected event, and the missing person is soon identified. Or a hiker who calls an emergency number to report an UFO and the call is cut off when the aliens catch the hiker... the police verify abduction, and air traffic control radars verify UFO. Then the aliens can make a second raid to return the abductee and again get out before fighter jets arrive - but not undetected. And if they figure the abductee is not useful, would they bother returning him?