Iain M. Banks only ever published one collection of short stories. Do we know if he left any short stories unpublished or have any come to light since his death?
As far as we know, there are no strictly unpublished short stories anywhere, no.
However, there are two short stories The Secret Courtyard and The Spheres, which were published only in a run of 500 chapbooks at the 2010 Novacon convention in Birmingham.
These are excerpts that were cut from Matter and Transition respectively, during the editing process for those novels.
The ISFDB has more details here.
Given the extremely limited print run, these are very hard to come across.
Additionally, there is a book title The Culture: Notes and Drawings, which is currently scheduled to be published in February 2020. This will contain, as the title suggests, some of Banks's drawings and further world-building notes and essays related to the Culture. Whilst we don't have full information about it, it is conceivable that it may contain further unpublished stories or excerpts, if they exist.