Subject: Moving light: what was that?

Some months ago, one morning, my mother as usually was opening all the louvers in the house. When she opened the ones in the bathroom, she saw a still bright light, that suddenly started to move away and reduce its brightness, until it was no more visible. As I haven't seen that thing, I can only report my mother's words.
What could be a rational hypothesis about it?

Re: Moving light: what was that?