I have wiped my gas cans and spray bottles with a rag when I have had a little spillage while filling them and / or using them. I've also wiped them when they get dusty from being in the garage. I can see George doing the same thing - not to remove fingerprints, but to keep things clean.
As for the lone piece of duct tape on the gas can, in the rather poor image we have from the document dump, it looks to not only be on the top, but it also looks to not be laying flat and smooth. This leads me to believe it is being used to cover the air vent. Most cans I have seen have a small air intake on the top opposite the pour spout. This air intake is normally closed when storing using a plastic flip cover or screw cap. On one of my cans the plastic tie that holds the cap to can is broken, meaning I have to be careful not to lose it.
It seems reasonable this is what happened with the Anthony's can - it was lost. So we have two scenarios. George lost the screw cap or otherwise noticed it missing, and used a piece of duct tape to cover it. No telling what happened to the roll. Second scenario is KC lost it while she had the cans, and used left-over tape from her deed to cover the hole to keep fumes and spillage from contaminating the trunk. She discarded the tape roll at some later point.
In either scenario, it is actually important to keep the can clean so that the tape will continue to stick as it is pulled off and replaced.