The red trace above is an absorbance spectrum from 12:55pm on 10/5/97. The trace below it is a portion of the reference spectrum for propane. Propane is clearly identified in the upper trace. It is easy to see how this incursion of propane could have slipped by the refinery's contractors without being detected - if they were monitoring for this compound in the region surrounding the "strongest absorbing peak" - as water vapor interference is very strong in this region. If this were the case, it would explain the "non-detect" they arrive at, and would also explain the very poor detection limits which they report at this time.
We cannot know for sure if this is what happened here, as neither the refinery nor its contractors have told us what the problem is. They have not even acknowledged that there is a problem, or that they missed this incursion of propane.