(Alert mode is when the crosshair turns orange)

I think I had it happen once when in game, but never took any notice of why.
I believe it has something to do with the object beneath the crosshair being invalid/disabled/deleted, but everything I've tried to do to make it happen hasn't worked. I don't really play the game either, so it may be something obvious that I don't know of.

Does anyone know when/how this happens? Would be of great help!

Thanks.