Hi @opux ,
It certainly looks like a “splash” mark but it makes you wonder how that would happen if the laptop was closed and something was spilled on it without affecting something else.
You would think that it would seep in and run. It looks more like the battery was exposed and then spilled on.
Are the marks just dis-colouration on the surface or are they “grooves” at all (bit hard to see in image) as if the battery case has been slightly eaten away by something acidic before being wiped off?
Is the battery hot to the touch when the laptop is charging or operating?
To me leaking lithium batteries smell like fruit mixed with a paint solvent (best as I can describe) but if it is hot then there would be hot plastic smells as well etc.
Since you say that everything looks clean and I hope dry, then create a Win 10 battery report, and check the Design capacity value against the Full Charge capacity value to determine the condition of the battery.