This is a personal choice thing for the DM and the players. It's definitely something that is way more possible as well with roll20 or other online situations. If they were doing it completely hidden without the DM seeing it, I'd call shenanigans on it. But because the DM is seeing it, I think this can be something that you discuss.
Start by talking with the DM. Don't immediately bring everyone in, there isn't a reason to and it makes the whole thing way more of a mess. When you talk about it, talk about how you understand that they want to do it for meta-gaming, but because only that one person is doing it that you're now suspicious of what is going on with that character and a meta-gaming more around that. Make it about your meta-gaming, not about disliking what the other player is doing, it takes lot of pressure off of the DM and make the conversation less awkward. Also, come with suggestions to this conversation. Maybe everyone rolls like that, even the players who haven't been. Sure, you still might be suspicious, but it'll be consistent or as @colmde suggests, maybe knowledge/insight/etc rolls are made in private, but visible actions are made in public and that's your suggestion.
After you and the DM have talked about possibly have a whole group discussion. This would again be bringing up your issues with it. It's about your hang-up with meta-gaming it. And when you do this, bring up the solution that you and the DM discussed already. If you have the buy-in from the DM, it'll likely be easier to get everyone to go along with it.
If there isn't a solution out of game, maybe you're right. Maybe there is something sneaky going on with that character and it's actually the DM's third cousin who they brought in as a ringer for a major plot twist later on. If that is the case, then kudos to you, you just figured it out and you get to have a great "I knew it" moment later in the game, and that's a ton of fun too.
Or, maybe you're wrong and that's just how the player prefers to roll. All word play intended. If that's the case, just roll with it and don't worry about it too much. If the story is still fun and the game is still good and most importantly, you're still having fun just enjoy the game and try your best to not let someone else specific choice bug you too much. It's probably just fine that they are doing it differently. You can always join in rolling that way and see if you like it better for the immersion yourself as a player.