Keyboard types by itself

I'm helping my sister and here is the issue she told me about:

When she puts the typing marker in a typing bar (i.e Google search bar) it automatically starts to type a certain key (to be exact "s") in a sequence. It doesn't stop no matter what. If typing option is available that's what happens.

Any one familiar with that issue? Is it a virus? For now I recommended her to check the keyboard circuit and to clean it (that recommendation I took from here about a bit different problem involving similar properties).

Thanks ahead