Bereavement visions are a well documented phenomenon.

Here's a quote from an interesting article on Psychcentral:

“In an interview published in the Boston Globe (April 20, 2014), [Bart] Ehrman argued that the belief in Jesus’s resurrection may have been founded on visual hallucinations among Jesus’s bereaved and grief-stricken disciples. Ehrman speculated that, ‘…the disciples had some kind of visionary experiences…and that these…led them to conclude that Jesus was still alive.’

Now, I am no position to support or refute Prof. Ehrman’s provocative hypothesis, but there is no question that after the death of a loved one (bereavement), visual hallucinations of the deceased are quite common. Sometimes, post-bereavement hallucinations may be part of a disordered grieving process, known variously as ‘pathological grief’ or ‘complicated grief’—a condition my colleagues have been investigating for many years, and which had been proposed as a new diagnostic category in psychiatry’s diagnostic manual, the DSM-5.

Though visual hallucinations usually are reported by a single individual, there are reports of ‘mass hallucinations’ following some traumatic events; in such contexts, clinicians often speak of ‘traumatic grief.’ A report from Singapore General Hospital noted that, following the massive tsunami tragedy in Thailand (2004), there were many accounts of ‘ghost sightings’ among survivors and rescuers who had lost loved ones. Some would-be rescuers were so frightened by these perceptions that they ceased their efforts. There may well be a cultural or religious contribution to the Thai experience, since many Thais believe that spirits can be put to rest only by relatives at the scene of the disaster.

There is a nice list of suggested readings and references at the end of the article if you're interested in learning more about these phenomenon.