Is there a method or tip to best memorize current enemy pieces' movements? I have started to play Chess again after a few years. It is not difficult to reproduce an opening or sticking with a plan during the middlegame, but the most difficult to me is to keep track of enemy pieces threats on open rows/diagonals/columns.

As an example during one game I've lost track of an open diagonal that has a bishop of the other player and no piece (white or black), and forget to take care of not letting pieces stay on that diagonal unprotected while I was thinking on something else (like a plan to stick with).

My question: Is there a tip or a known method of pros like "memorizing/imagine colors" or create a "mental X mark" inside all those squares that makes part of a enemy movement possibility?

Sometimes I get caught by those pieces that are full aggressive (not protecting anyone)...
What you are talking about is called "board vision". Googling "chess board vision" will get you a ton of stuff. 

One thing that may help. On looking at an enemy piece (or one of your own) do not ask "where is it?" but ask directly "what is it doing?". For example, that Bishop is controlling that diagonal, which is annoying because I would like to use some of those squares. The battle for control of an important square might involve pieces close to or far away from that square, but they should be linked in your mind as participants in the same struggle. Psychologists call this chunking. You can remember things about a group of pieces more easily than about the individual pieces.