In relation to a question: Are there muggle ghosts in the Wizarding World?, it says that only wizards may become ghosts. But, can a Muggle see a (wizard) ghost?
Ghosts may roam the places they used to inhabit during their lives.
Albeit strange, there may have been a wizard that lived among Muggles, and enjoyed it, and happened to become a ghost in a Muggle-populated area. Or perhaps some of the witches that were burnt at the stake became ghosts and still haunt Muggle villages.
Whatever, can Muggles see ghosts? If yes, I guess that would mean a potential risk for the wizarding world's secrecy. What measures has the Ministry taken to avoid this?
Of course, no one's going to believe you for screaming about ghosts in public (like the Muggle ministry), but ideally that should be avoided for the Muggle masses (imagine a ghost that suddenly decides to display itself in front of as many Muggles as possible!)