Do them individually in a place where another "flare" won't cause a fire and take the other cells, your BC-6, or YOUR HOUSE with it. I'd examine them very carefully for signs of some residue.
Here's a thought... Have a dog? Line the cells up in some plastic wrap about a foot away from each other. Lead the dog over to have him sniff them. He may spend more time sniffing something "interesting".
Off Topic True story: In testing a dog's ability to sniff cancer, doctors took a group of 6 people and tested them for cancer to get 6 known negatives. Then took one person that is known to have cancer. They all exhaled through some tubes with gauze in them. The dog picked up the known positive, no problem, but would not leave one of the other samples alone. They took that sample and re-tested the person again. After a few days, they tested positive for cancer.
If a dog can do that, it can certainly smell lithium ion flame residue.