The RAF was scrambled to locate a distress call - after local searches left the RNLI baffled.

A distress signal usually activated when boats sink was picked up by the RNLI at Gravesend yesterday morning.

But rescue teams were unable to track down the vessel.

A Sea King was deployed to locate the satellite-guided safety beacon - which was eventually traced to Darent Valley Hospital.

An RNLI spokesman admitted it had no idea what set off the Epurb, as the tracking device is called.

The RNLI was first called out after the Epurb warning at 10am.

When lifeboats could not locate the source of the signal they called in the Sea King 125, a search and rescue helicopter based at RAF Wittering, in Peterborough.

The aircraft has specialised locating equipment and found the signal at Darent Valley Hospital, but the RNLI said it did not know what set off the signal.

An RNLI spokesman said: "he Epurb had been activated by mistake and the distress signal was found near the hospital.

"We don’t know what set it off."

The RNLI returned at just before noon after the Epurb was located.