In the Cochrane Handbook it is noted that "... failure to identify trials reported in conference proceedings and other grey literature might affect the results of a systematic review." (In: Cochrane Handbook - 4.S1 Technical Supplement to Chapter 4: Searching for and selecting studies)
Campbell Collaboration: "All reviews must include a systematic search for unpublished reports" (In: Campbell Collaboration - What is a systematic review?)
Key sources of unpublished systematic reviews include:
See the Library's Systematic and Systematic-like Review Toolkit guide for comprehensive information on developing systematic reviews.