Maybe your team, some people use IntelliJ and other people use Metals.
While IntelliJ shows a bunch of warnings out of the box in their IDE, Metals requires some settings (like unused imports).

Metals' warnings depends on Scala compilers warnings, while IntelliJ uses its own typechecker and shows them out of the box.
Which means, we have to add some scalacOptions to scala compiler such as scalacOptions ++= Seq( "-Ywarn-unused:imports") (for unused imports).

For Scala2, this blog post might be a good read Make Your Scala Compiler Work Harder | by Dick Wall | Life at Hopper | Medium