If that's from one item, it's around 600w so you're looking for something like a computer tower or a heated floor mat under a tiled floor? The only way to definitely track it down is to monitor each circuit from your consumer unit with a power monitor.
The only things left on is 2 Alexa devices , fridge/freezer, broadband and deco Wi-Fi boasters. Also the robot hoover
It’s strange. Some days I have 0.5kw used by lunch time and other days it’s 3.5kw before we even get up. Nothing is left on overnight.