Can Muggles see ghosts? If so, what measures are taken to prevent that?

Question

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!)

Answer 1

As seen in Fantastic Beasts and Where to Find Them, magical creatures like dragons and unicorns were often spotted by Muggles in the past, which is why Muggles have these creatures in stories or art. Similarly, Muggles have many ghost stories, which undoubtedly come from Muggles actually spotting ghosts somewhere.

There is also a Spirit Division in the Department for the Regulation and Control of Magical Creatures in the Ministry of Magic, which most likely has laws preventing ghosts from revealing themselves to Muggles, and punishments for ghosts who do not obey these laws (Moaning Myrtle, for example, who had stalked a girl who had teased her just before she died for many years, had been forced by the Ministry to return to her bathroom at Hogwarts). Obliviators could clean up after a ghost sighting by a large group of Muggles, and it is suggested in Philosopher's Stone that ghosts may have the ability to become invisible, so they could hide from Muggles if they wished.

In other words, yes, Muggles can see ghosts, but the occasion would be unlikely and they would likely not remember the occurrence if they did.

EDIT: Actually, I was completely incorrect. Muggles cannot see ghosts. This is stated explicitly by J. K. Rowling herself on Pottermore, Book 2, Chapter 14, Moment 1, in the bonus material concerning ghosts.

Answer 2

No, a JKR Pottermore writing says Muggles cannot see ghosts.

The writing by J.K. Rowling on Pottermore about ghosts states that Muggles might feel that a haunted place is cold or creepy, but won’t see ghosts clearly like wizards can.

Witches and wizards are much more susceptible to what Muggles call paranormal activity, and will see (and hear) ghosts plainly where a Muggle might only feel that a haunted place is cold or ‘creepy’. Muggles who insist that they see ghosts in perfect focus are either a) lying or b) wizards showing off – and in flagrant breach of the International Statute of Secrecy.
_{^{Ghosts (Pottermore)}}

From this we see that it’s impossible for a Muggle to see a ghost clearly, and any Muggles who claim they can are either liars or wizards in violation of the Statute of Secrecy. So, if a ghost decides to haunt a mass of Muggles, all they might feel is cold and creeped out - nothing that would make them suspect magic.

However, it’s possible, though not clear, that Muggles may feel it if a ghost passed through them or partially passed through them.

Should part or all of a ghost pass through a living creature, the latter will experience a freezing sensation as though they have been plunged into ice-cold water.
_{^{Ghosts (Pottermore)}}

Since it’s said that “living creatures”, not just wizards, will experience the freezing sensation, it’s possible that Muggles would as well. Again, this could be explained non-magically, though it seems likely that a ghost that kept passing through Muggles on purpose would be punished by the Ministry like Myrtle was.