Not native Signal but it happens with Signal forks that I install after adding repository to F-Droid, I have had a notification of a Signal update, even though I'm not using native Signal.
I disable that notification in the phone app settings and wait for an F-Droid notification of an update to install.
I'm completely open to hearing why the Signal update notification is a concern. I don't worry about it but you may know something that I am not seeing.