Upon removing the cap from the tank, I noticed the inner part of the cap (the sealing part) was left in the neck of the tank. It separated from the outer screw-off portion for some unknown reason. I pulled it out of the neck with pliers. I don't recall if this was an OEM cap or an aftermarket cap, but I know that I replaced it years ago on a preventive maintenance basis.

LUCKILY, I had a spare cap on hand, the one that I previously replaced. It's usable and avoided downtime since a cap is necessary.

I performed a quick online search and discovered this is a common problem for plastic caps on many car models. So it would be a good idea to have a spare.