People typing whatever is the latest fad, such as "rn', " tbh", etc.

Disheveled hair. If you have long(ish) hair and you're going out in public, at least drag a comb through it so you don't look like a bed-head.

Their taste in music. Sorry but if Pink Floyd is your favorite band i have to assume you're boring

Whether people read as a hobby or not. As it implies a type of interest into the world around them.

And as a lesser second, what they read.

For being british.

If I told you, it wouldn't be secret.

Why yes I am a clever little scamp!

Eating meat although they're fully aware, that we have to shift completely away of that (GHG emissions, land-use), and then blame the government that they need to regulate this more.

Yes more government regulations would be great, but it's one of the few individual things that have effect, if everyone would think similar. And a vegan or mostly vegan diet is not really worse in taste and likely more healthy as well... Eating meat is not sustainable (nor morally justifieable), it should be a thing of the past...

Using proprietary chat apps like Discord, Telegram, Slack, LINE, Meta’s WhatsApp / Messenger. Still judging on apps that require a SIM & mobile OS (like Android) primary device like Signal… or an expensive chat protocol like Matrix.

Hosting your code & bug tracker with a propietary forge like Microsoft GitHub when you say you support open source—but don’t even bother to apply the same mentality to your own project.

…Oh, the question was “secretly”.