Thanks for the comments. It hadn't actually occurred to me that it could be condensation! Where it collects indicates that it could be condensation and because the blinds are left closed, I may not have noticed it before.
I did get ChatGPT to decode the text of the label and it was of some use, pasting below for info, not all spot on but may be of use for others.
The label indicates that the window is a Dometic S5 window, which is a double-glazed acrylic window with a roller blind and a fly screen.
The window has a DOT-769 code, which means it is a Seitz window, a brand that is part of Dometic Group.
The window has an AS-5 code, which means it is a sliding window, which can be opened and closed horizontally.
The window has a M-50G code, which means it has a grey frame and a 50 mm wall thickness.
The window has a V-X/B(E1)43R-001748 code, which is the ECE approval number, which indicates that the window complies with the European regulations for vehicle safety.
The window has a PES S/Q/>PMMA< code, which means it is made of polyester (PES) and polymethyl methacrylate (PMMA), which are synthetic materials that are durable and transparent.
The window has an ADP7M20490X0500 code, which is the window size, which means the window has a width of 490 mm and a height of 500 mm.
I will try and establish if the water is condensation before going any further, perhaps get the wife to see if we can do a leak test with the hose.