I'm sorry to say that I don't have a link handy. I'd hope my comments come off honest enough that you'd believe me, but it's fair that you'd want to verify for yourself.
I'm sure you can eventually track down n0thing and other former CS 1.6 pros referencing the practice. These videos -- clips from old Twitch streams and YT videos -- resurface from time to time, and then disappear.
In n0thing's case, he explicitly stated that (a) he cheated in pugs or even scrims at times to visualize player positioning and movement, and (b) pros regularly did this to be able to visualize opponent position, particularly for wallbangs. He also insisted that he never and would never do it in a tournament.