Maybe I just forgot about this (it feels vaguely familiar), but today I was pretty surprised by it.
In Sapienza, I was checking if I could snipe Silvio from the clock tower by playing the grammophone, etc to get him up to his mother's room.
However, I know that he faints at a spot where you cant get an angle on him, so I intentionally didn't do the bit that makes him faint (starting the fan to make his mother's rocking chair move), and tried placing a coin for him to fetch at the right place.
Turns out if he gets this far in the sequence and doesn't faint (and maybe if someone wakes him up, haven't checked), he'll go directly for the seaplane escape (so no need to scare him once to go to the science lab, and again once he's there to trigger the escape).
Desperately I grabbed my sniper rifle and ran down to the flower truck for an angle on the plane, but not even by shooting it with all the bullets in my Sieger did I do enough damage to destroy it (realistic, but I've managed with a shotgun before, so I figured I'd try).
Anyways, I guess I just found out you can make him escape by seaplane without doing the double scare.