Every single person has "something to hide", and that's normal. It's normal to not want your messages snooped through. It doesn't mean you're a criminal, or even computer-saavy.
Like the Casio watches, travelling to Syria, using Tor, Protonmail, etc…
When it is better in reality to have a regular watch, a Gmail with encrypted .zip files or whatever, etc.
It does not mean you are a criminal if you have that Casio watch, but if you have this, plus encrypted emails, plus travel to some countries as a tourist, you are almost certain to put yourself in trouble for nothing, while you tried to protect yourself.
And if you are a criminal, you will put yourself in trouble too, also for nothing, while you tried to protect yourself.
This was the basis of Xkeyscore, and all of that to say that Signal is one very good signal that the person may be interesting.