It is indeed a difficult problem. But it is a fact of physics that hot (thus energetic) He atoms at higher concentrations than found in the atmosphere (equalisation of partial pressures etc etc) will leak out of a reactor faster than cooler ambient He can leak in. So anything over background is an anomaly of interest.
Yes of course Alan. My understanding is that the concentrations found in the He experiments were hundreds of times lower than background concentrations - or in some cases (the experimental conditions varied) 100s of times lower than plausible lab atmosphere background concentrations (at times) from He used in the lab in other experiments. Single-time (or even single-location - unless colocated) measurement of He in a lab would not be enough to determine these, unless it could be certain that no lab equipment used He.
understanding all the He experiments is complex - we would need to engage with that complexity and all of the details now to redo the analysis done at the time, which convinced some, but not others. For me who it convinces is not the point (I am not a fan of PR) but the details are the point.