If you search hard enough you’ll find where they copied the code from.

Sometimes it not hard at all. SO and GH are the most probables

In a source code from my boss I once found a random function, which was not used and was commented with "//found this on Google, don't need it now, but might be useful"

Maybe not copied from but the same solution because human brains think in similar ways. Imagine making a console app that displays a tree without using for(int i=0; i<10; i++)
because it has been done before and they might think you have copied it from the internet...