[SPOILER ALERT] Bug or not? Displaying all targets sometimes

PC, Professional difficulty.
When I was first time climbing radio tower, I got this sniper guy saying regularly “I noticed our man on radio tower” or something like that, and at that point I could see all targets red in detective mode.
I couldn’t repeat it later on.
Just now, I’m playing, I started undercover as rave freak at the entrance, subdued guard in green spitfire jacket and wonder around, now I notice I can see all the targets again in detective mode!

Here it is:

So why does it happen and when?
Or is it a bug?

They are marked in all starting locations except the Bus Station. That is intentional.


Thanks, I never guessed it