Can passenger airliners hover completely motionless in the air?
In theory they can stop moving relative to someone standing on the ground. In practice this does not occur. 

Aircraft fly through the air and, yes, below a certain airspeed, an aircraft will stall. Let's take a super-simple example to explain the situation: 

You are standing on the ground on a very breezy day. It's quite windy on the ground, and blowing even harder at altitude. For the sake of this example, let's say at 15,000 ft. There is a 150Kts wind from the west. A small airliner is slated to go west at 15,000 ft and levels off but slows down to a cruise speed of 150Kts. The airplane is still moving through the air at 150 Kts, but the air is moving in the opposite direction at the same speed. In terms of a fixed point on the ground the airplane does not move. 

However, big jets are capable of flying faster than even the biggest practical headwinds. You can, however, observe this in small aircraft on a gusty day (I have flown a PA-28 backwards in this scenario). Here is a video of someone doing it in a slow plane.

What is likely occurring is that a plane that is very high up and very far away appears not to be moving due to its small size in the sky and lack of surrounding points of reference.