Chris Borasinski's mother Janina was admitted to Croydon University Hospital on Saturday and was later discharged.
After failing to return home, Ms Borasinski was reported missing but was in fact stuck in a hospital toilet for hours, her son said.
Croydon Health Services NHS Trust has launched an investigation.
Mr Boranksi welcomed the move saying it would mean systems could be changed so "they are not going to lose any more patients".
He added: "It beggars belief that this could have happened."
Ms Borasinski was admitted to the hospital after becoming distressed at home but, following tests, she was deemed well enough to be discharged.
Her son, who lives in Germany, flew into the country after learning she was unwell and made his way to the hospital.
When he arrived at 21:15 BST he was told by  staff that his mother had been discharged.
Mr Borasinski said: "I said that she probably would not have been able to take herself home as she had no money, no travel card, and she wasn't appropriately dressed as she in came in by ambulance.
"The nurse said as she was 84 they would have organised transport home."
He tried to contact his mother and when he arrived at her home in Waddon, Croydon, found she was not there.
Mr Borasinski phoned the police to report that his mother missing and officers and hospital staff began a search.
At about 08:30 BST on Sunday Mr Borasinski received a call to inform him his mother had been found in a toilet cubicle at the hospital.
After she was found Ms Borasinski told her son she was quite weak when she was admitted and once she entered the cubicle she was unable to leave or trigger the alarm.
His mother has since been readmitted and is currently undergoing further tests.
Following the incident, first reported in the Croydon Advertiser, a spokesperson for Croydon Health Services NHS Trust said: "We are very sorry for the distress Mrs Borasinski experienced over the weekend and we have apologised to her and her family.
"An investigation is taking place to determine exactly what happened.
"We are speaking with the family and our findings will be shared with them first."
A hospital has apologised after an 84-year-old woman was found locked in a toilet cubicle 12 hours after being reported missing.