Neighbour Sue Novell's daughter Katie found the lady sitting outside Estrella Rest Home, Main Road, in the middle of the night.

She had somehow made her way out the front door, but could not get back in – and it was about 15 minutes before there was any response from inside the home.

The home has changed ownership since the incident.

Mrs Novell said: "I think it is quite disgusting to think somebody can get out. There are obviously no alarms on the site, no protection for them and the doors are not locked.

"We don't know how long she was there – she didn't know who or where she was and she was very cold and upset."

The incident, which happened on July 25, is now under investigation by the West Sussex County Council social services.

Depending on the outcome, the Commission for Social Care Inspection may become involved.

Mrs Novell has lived on her farm next to the home for the past 17 years. When her father was alive he used to bring patients back to the bungalow for a cup of tea.

She said: "I'm not going to let it rest this time. It has happened too many times. These people need to be looked after."

Estrella changed ownership on Friday, with new owners LRH Homes insisting new policies are in place to ensure incidents of this nature do not happen again.

Managing director Jag Grewal said: "The previous ownership policies were to have staff sleeping in. What we have done is to ensure they are not sleeping, but there are staff awake.

"It is sad it has happened, and it is important to ensure it does not happen again.

"The incident happened under the previous ownership. It is sad but these things can occur and it is important to learn lessons."

A county council spokesman said: "A concern has been reported and this will be looked into."