The Sulphuric acid (H2SO4) is a liquid. Normally a battery uses a
dilution of around 25% acid in water.  As you discharge the battery
the acid content is converted to Lead Sulphate, and regenerated as
Sulphuric acid when charging. If you overcharge the water content is
electrolysed to hydrogen and oxygen gases which escape (that's why
they say don't charge in enclosed space). A tiny amount of the acid
may be lost as spray but virtually all of it will still be in the
battery, but probably absorbed in the plates and not visible. Adding
water to the acid generates a lot of heat, enough to spit some back
out, so add a little at a time, at arms length and wear gloves and
face protection. Concentrated battery acid will take the skin off.

....so the acid was still there, just in the plates

rK