Dave, 56, of Cromwell Road, Southsea, disappeared while on a week-long holiday in Turkey on July 2 after leaving his hotel and planning on walking to a ghost town popular with tourists.
Gavin Thomas, his nephew, said that the family had received an email on Sunday from a holidaymaker in Crete who claimed to have seen Dave.
The tourist claimed that the man was acting ‘incredibly erratically’ and had been taken into police custody.
The Foreign and Commonwealth Office (FCO) and Turkish Local Consulate have determined that the person seen on the Greek island, which is a 22 hour drive 570 miles away from where Dave was last seen, is not the missing Portsmouth man.
Mr Thomas said: ‘This is of course a huge disappointment and as a family it is our first experience of a "sighting" in this missing person case.
‘We accept that by sharing David's profile & image as widely as possible, sighting's like this are likely - and all know that this awareness is needed in order to help find Dave, even if negative results like this seem heartbreaking.’
The person seen acting erratically in Crete was confirmed to be a German national according to Mr Thomas and that Dave’s details have been passed onto the Greek Authorities.
Search operations are still being carried out in Turkey but one of the Non-Governmental Organisations (NGO) involved AKUT have said they will not be able to continue without new information.
Mr Thomas said: ‘As hard as it is to admit, we think we really have done as much as possible locally.
‘The AKUT team on Saturday confirmed they can’t continue to search without further information, and so the search is now mainly being done by the police/Jandarma and I am sure on a smaller scale.
‘We are extremely worried that this means the intensity of searching will drop off and progress will now be slow to none.’
A Go-Fund-Me page has been set up to help with the search – it can be found here.