If the fluid in question was fuel, you have a petrol leak and a big opportunity for the engine to catch fire. If it was water, it could be condensation from the atmosphere or perhaps even residual water from when you last washed the car. If not, the water could have come from the car’s cooling system, at which point you have a coolant leak and that needs to be fixed before the engine overheats. If the fluid was oil, suspicions would be a leaking rocker cover gasket.

Fundamentally, there shouldn’t be any unexplained fluids in the engine bay or on the engine, and if there are, something is probably leaking. By the way, the fluid you notice could have actually been the cause of the misfire; spark plugs can misfire if they’re in contact with water or, especially, oil.

My money’s on the fluid, indeed, being oil. The park plugs on this engine are located deep inside the rocker cover. Any leak from the gasket on that cover allows the oil to pool in the spark plug tube, which means it’s a fair chance this is the cause of your misfire, too.

It won't be the sump itself disintegrating, but my guess is that somebody who has changed the oil in the past has over-tightened the drain plug when they've replaced it. That stretches the threads on what is often a relatively soft metal and, as a result, the threads have broken away from the rest of the oil pan as you've tightened the plug.

This is actually more common than you think and happens because people don't stop to think about how much force they use when they replace the plug. While it's true that you don't want the plug undoing itself while you're driving, it's also the case that all that sump plug has to do is keep the oil inside the engine. It serves no other purpose and simply doesn't need to be too tight. Sometimes, home mechanics ignore the need to replace the crush washer on this plug with the result that they then notice an oil a leak a few days later. That's when they'll reach for the big spanner and try to tighten the sump plug some more to stop the leak. And that's when the threads break away and they're in the position you now find yourself.

The solution is to get the vehicle to a workshop (you may have to tow it as it can't be driven with no oil in the engine) which should be able to drill the hole in the sump to the next over-size and fit a slightly larger, new sump plug. It's tricky though, as you don't want to allow swarf (little metal filings from the drilling process) into the engine where they'll damage those expensive internal parts.