Theft is complete when the dishonest person appropriates property with intent to deprive the owner of it permanently. The moment the shoplifter takes the item off the shelf he is guilty, but the problem for the police is proving, from that mere fact, that there was an intent to steal. But there may be other circumstances which prove the intent, such as the man hiding what they are doing, looking around, being equipped with articles for use in theft such as a coat with a hidden pocket and so on.
It is simply practice to wait for the person to leave without paying because their leaving shows intent to permanently deprive. The person could be arrested inside, if there were reasonable grounds for believing they were committing theft.