You can keep all your scary movies. To me one of the most frightening things I can hear is someone ask very quietly "Is this stuff under the sink supposed to be wet?".
We'd just about convinced ourselves that the out of place water was caused by a leak from a pack of wet wipes, when I discovered that a plastic box that was supposed to be full of cleaning materials was also full of water.
To cut a short story shorter, it turned out that the washer above wasn't doing what it should. No matter how tight I made the connection (within common sense "Don't tighten it until it breaks" limits) it still dripped. About a drop every minute. More than enough to fill a big plastic box over a period of a few months.
I'm quite proud of what I did next. One approach would have been to remove the hose from the back of the washing machine and fit a the spare one I keep for such occasions. A fun packed exercise which would have involved dismantling chunks of the kitchen. And puddles. Another approach is to take the washer off the spare hose and fit it to the existing one. End of leak.