Looks like you have a little flat roof over the bay.
They usually last 30 years, sometimes 50
I presume you can't get up there to look at it
So ask around for a recommended local roofer or someone with a drone
You might be able to poke a selfie stick out of the window to see if the gutter is spilling or if there is a defect in the detailing above the window.
The gutter joints look poor to me and the damage to the render may indicate spilling or leaking gutters. Watch them during and just after rain.
See if you can borrow a pair of binoculars, or buy the large Lidl ones next time they come up. For the same money, the large Zeiss shape will give a better quality image that the tube-like roof prism design.