Despite no visible gel product, the melting curve peak in the 5x primer negative control likely indicates non-specific amplification or primer-dimer formation, detectable by the sensitive melting curve analysis but not by the less sensitive gel electrophoresis. This could be due to incomplete PCR reactions, primer-dimer artifacts from high primer concentration, or fluorescent dye binding to small, non-specific products. To resolve this, re-evaluate primer design, optimize PCR conditions (including primer concentration and annealing temperature), use more sensitive detection methods, and include additional controls to distinguish specific amplification from artifacts.

Nucleic acid bands can fade or vanish due to diffusion or degradation when running an agarose gel for an extended time. To avoid this, it's crucial to monitor the progress of the gel electrophoresis and stop the run at the appropriate time.