There was an individual that I kept in my contacts, you see, for the the sole purpose that if he ever called me, I'd know to let it go to voicemail. We had been close long ago, but he stopped living in consensus reality and wasn't interested in treatment. I considered him disturbing but not immediately dangerous, just someone I didn't want to reconnect with.
When I installed Signal, he got the notification that I had done so, and immediately messaged me, along the lines of "Oh hey, you still exist! And I guess by the timing of this install, you must be at [security-focused event] this weekend, yeah? Hey let me tell you about my latest harebrained scheme..."
I understand that Signal needs to do that sort of connection to work behind the scenes, but they don't need to generate an alert on the guy's lock screen about me.