The mobile phone was found underneath a washbasin in a Starbucks in Lancaster, California, as his mother used the facilities.
The video function had been left running and the woman said she didn’t realise she was being filmed until her son said: ‘Mommy, why is there a phone under there?’
The woman said: ‘My son used the restroom first and I used the restroom after him,’ she told TV network KTLA 5.
‘I noticed he was staring at the sink a little bit perplexed. I asked him what was wrong.’ That is when the little boy told her what he had seen.
When the mother looked she found the phone propped up with paper towels.
It was unclear how long the phone had been recording and whether or not it was being streamed to the internet.
A spokesman for Starbucks said: ‘We are disturbed by this incident and take our obligation to provide a safe environment for our customers and partners (employees) very seriously.
‘Our store partners took swift action as soon as they became aware of the incident by immediately alerting the Sheriff’s Department of the situation.’