Suppose, hypothetically, that you had been passing a very scruffy old Datsun pickup truck parked in one of those soon-to-be developed areas where parking restrictions are not enforced, which tend to collect old and possibly abandoned vehicles.

(I parked my truck somewhere like this near Kings Cross, but it was a well dodgy area even by London standards and was eventually vandalised and stolen)

Suppose you rather liked those trucks and were saddened to see this one so neglected.

Suppose further that you had some home made rust inhibitor (hypothetically, a mix of diesel, hydraulic and sunflower oil) left over from treating your own car.

Suppose on the third passing, late at night, you cracked and skooshed some over the bonnet and down the grille in front of the windscreen (a rot trap on these).

Suppose this happened in the UK, and not some corner of a foreign field where the law would be different

If you got caught (video?) what might they charge you with? Its clearly an invasion of privacy, but getting a malicious damage charge to stick might be difficult.

(A Taiwanese court would be completely baffled by the idea of preventative maintenance so it might be the only option.)