Nuthatch in the Scottish Highlands?

So I was volunteering for the RSPB Loch Garten in late August this year and heared a call that sounded distinctly like a nuthatch (DOO-WIP DOO-WIP). RSPB staff assured me that no nuthatches live in the Scottish highlands, as did most ID guides. I heared this call clearly on several occasions in different locations in the Scottish highlands though always in pine forest. Any idea what it could be?