I think there are several signals that would indicate concern and should result in an escalating level of planning and review. My first signal would be volume of hydrogen involved including the source tank and any reaction and intermediary containers and piping systems. Escalating factors would include piping and reaction vessel pressures, flammable/explosive gas mixtures, and process variables. The use of any gas pressure vessel in research causes a red flag for me, regardless of the gas. Very few research scientists are conversant in pressure vessel design issues, particularly outside of mechanical and materials engineering departments. Most of us have probably been witness to burst glass reaction vessels on the order of half a liter, which can cause significant personal injury. I'd be concerned with explosive gas mixtures below that volume for sure -- considerably less concerned if it was in a latex balloon.