I have the same setting, but vim automatically sets `noexpandtab` when editing a file named Makefile. I don't have any explicit setting to do that, nor any Makefile-related plugin.
Yes, you can see this in the make filetype plugin in Vim’s $VIMRUNTIME directory, which is usually
/usr/share/vim/vim81
which contains all of the plugins, syntax definition files, etc. that come with Vim. The file for make is located at $VIMRUNTIME/ftplugin/make.vim.
It’s pretty straight forward to override any default behavior you’d like to change; this is a great article on how all of this works [1].