It was most likely just caterpillars.
"The leaves are eaten by the caterpillars of several species of moth, including the sycamore moth, the mocha, the maple pug, the small yellow wave, the prominent and the maple prominent."
-
www.woodlandtrust.org.uk/visiting-woods/...e-trees/field-maple/
That's if you live in the UK. If you live somewhere else, it could have been other species.
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