Occasionally, supermarket chains are targets of extortionists who tamper with goods or food - for example, the Tylenol case in the US and a recent Australian case where criminals contaminated toothpaste and mouthwash with arsenic. In some states in the US, alcohol is sold in supermarkets and is popular with thieves. The arrest rate has not changed over the past 10 years. Several systems are available, most notably electromagnetic, low frequency and high frequency. The tags can be disguised as bar codes or otherwise hidden.
Some supermarkets install sensors at each checkout aisle.
After groceries are rung up, the customer walks between two arched steel bars, and any tagged items hidden on the person will cause an alarm to sound. The cashier can then ask the customer to step back and walk through again, and suggest that the person has forgotten something he or she intended to pay for. Other supermarket operators put EAS sensors at the store's exit. This is obviously cheaper than placing them in each aisle, and does not require each checkout to be rewired. One of the most recent innovations in the technology is systems which trigger only when activated by alarm sensors or other signals.