It's difficult to say but almost invariably this is when screen has been fitted before tray is sealed.
There is a gap behind shower screen profile that isn't sealed and any attempt to seal externally will just make a larger area for water to puddle and make it worse.

I understand that the thread is one year old, but I'll still provide a friendly casual reply to your post about the leak coming from the shower onto the skirting board.
Based on the watermarks on the skirting board, it does indicate a leakage issue that needs attention. While it's challenging to determine the exact source without further investigation, it's advisable to get a professional plumber involved to assess and fix the problem accurately.
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A leak coming from the shower onto the skirting board can be a significant issue, leading to water damage and potential mold growth. It’s important to address the problem promptly to prevent further damage to your bathroom and surrounding areas. Regular maintenance and inspection of your plumbing and shower system can help prevent such issues. For comprehensive care, consider scheduling AC Maintenance to ensure that all aspects of your home's systems are functioning properly. Addressing leaks early can save you from more costly repairs down the line.