I opened it with a metal spudger -- not the best tool to be digging around inside a device packed with batteries, and I don't recommend it -- but it was quick, and once open I immediately saw what was wrong.
A loose wire. A loose wire right near to metalwork.
A loose wire that sparked into life.
Defective '5,000,000mAh' power bank
Safety warning: Don't mess around with defecting electronic devices unless you know what you are doing. Rechargeable cells contain a lot of power, and can cause fires that are tricky to put out.
If that wire had made a connection with the metalwork that lasted more than a fraction of a second, something would have given way. Maybe the wire would have melted, or perhaps the solder joints would have melted.
But it could have caused a fire and could even have caused the batteries to overheat, potentially resulting in a fire.
I took the plastic wrap off one of the batteries to examine it, and it looks like it's a poorly made cell that is showing signs of leakage or corrosion on the top.
I reconnected the loose wire to the circuit board and the power bank started working, confirming that this was the problem and that the batteries held a charge.
The argument could be made that this is a defective product rather than a dangerous one, but a fire is a fire.
What's ironic is that this power bank was only a few pounds cheaper than something made by a reputable company. These cheap power banks with their vastly inflated capacity ratings are flooding sales websites, duping customers into thinking they are buying a quality product.
As is always the case with dangerous or defective products sold in the UK, I liaise with UK trading standards officers to get the offending products removed from sale.
How to test chargers and power banks to make sure they don't blow up your expensive smartphone