most likely a small bit of water being forced in through the seals of the window when pressure from a gust of wind presses up against it causing a slight gap.
It looks like the wind blew rain water into the narrow crack along the bottom of your window, then it dribbled down the wall. The force of the wind was so great that even the tiniest opening might get water forced through it. Hopefully this will leave no marks once it dries out. You might get some weatherstripping for the window to seal it tighter for next time.
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most likely a small bit of water being forced in through the seals of the window when pressure from a gust of wind presses up against it causing a slight gap.
Looks like ectoplasm.
[Condensation.](https://youtu.be/CQGW46B6x_4)
Seal your windows.
It looks like the wind blew rain water into the narrow crack along the bottom of your window, then it dribbled down the wall. The force of the wind was so great that even the tiniest opening might get water forced through it. Hopefully this will leave no marks once it dries out. You might get some weatherstripping for the window to seal it tighter for next time.