If he does have 3/4 gravel bed under the slab, he is likely to see several CFM out -- at least for part of the years.
Only if the sump pit is plastic/metal lined ( like a pal without any hole) ... beside the single hole for the french drain... could it be considered normal for the sump pit to read near 0 CFM most of the time.
Even then, if the sump pit dried off, the pit is likely to show a measurable amount of gas flowing. (not necessary air... but water vapour)
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