ATM-skimming is a form of theft in which people attach devices to record one’s card details electronically and film one’s PIN number being entered into a bank’s hole-in-the-wall machine.
One relatively cheap technique to prevent this is to equip each ATM with a small recessed lamp, shining brightly and obliquely onto the front plate of the ATM (each machine might even have the lamp located in a slightly different position).
This would cast shadows from any protruding parts of the plate. A line would be engraved to correspond with the edges of these shadows.
Users would then immediately be aware of the presence of anything added to the ATM, including illegal card readers, cameras, etc.