Requirements may be derived from a number of sources. Determining the source of a given requirement is known as pre-requirements tracing. Typically, some requirements appear that have no clear source, yet stakeholders will attest to the necessity of these requirements. However, such requirements are likely to be based on tacit or tacit-like knowledge embedded in the problem domain. A tool called Prospect that retrospectively identifies pre-requirement traces is presented. This tracing is achieved by working backwards from requirements to the documented records of the elicitation process, such as interview transcripts or ethnographic reports. A vector-space technique, latent semantic analysis, is shown to be useful to perform pre-requirements tracing. The identification of badly sourced requirements naturally leads to the inference that further investigation of these requirements is necessary, whether or not the requirements turn out to be based on tacit knowledge.