Originally Posted by
Land-of-Miles
I haven't encountered this at T5 before but it is a regular feature at T3.
The BAA security staff have been instructed to deposit everything in their own bag to ensure that it is completely airtight. One of the outcomes of the initial security screening triggered either at random or due to what is seen on the scanner is to perform an electronic "sniff" test of the contents of the liquid bag. The rationale for this should be obvious. However the accuracy of this test depends entirely upon the bag containing the liquids being airtight. Small tears or perforations in the bag can impact the test. the only way to be sure the bag is completely airtight is to use a brand new one supplied by BAA.
That is the explanation I have had when I asked nicely. I have no reason to disbelieve it and it does have clear internal logic.
There's no logic to it. If a plastic bag can be so airtight as to catch volatile fluids in the few minutes between transferring into it and a sniff test, then one would assume a liquid container in the bag could be also so airtight to prevent release of anything that the machine could sniff out.