I installed Signal a few years ago, and pretty much never touched it again. Today I open the app and closed it. After a few minutes, there were a lot of hello ‘messages which got me intrigued. It seems when I opened the app, all my contacts using Signal got a notification! What gives? Obviously uninstalled it straightaway...
By default Signal notifies users when someone in their contacts starts using Signal. You may have installed it before that feature existed so it triggered when you opened it.
Isn't that what all chat apps do? Telegram as t least, and I keep getting notifications of people joining for names I've never ever heard before, so seems like they were a bit overeager even.