Why did Peeves think a ghost could become invisible?

Question

In PS (16) the Trio, headed to the Forbidden Corridor under the invisibility cloak, encounters Peeves, who hears them and says

Know you're there, even if I can't see you. Are you ghoulie or ghostie or wee student beastie?

and Harry decides to impersonate the Bloody Baron and answers

the Bloody Baron has his own reason for being invisible.

Now, a witch/wizard can, of course, hide through magic. What about ghosts and ghouls?

According to logic, I would say that only someone (something) physical and material can become (be made) invisible in the sense that it is not visible but is still physically there. However, on Pottermore ghosts are characterized as

transparent, three-dimensional imprint of a deceased witch or wizard, which continues to exist in the mortal world

and

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.

So ghosts are specters that can either be seen (as pale, translucent versions of their pre-death selves) or only felt. Who is able to see them could, at most, not see them in perfect focus. Throughout the books, it appears that Peeves - similar to witches/wizards - sees them and interacts with them, as implied by Harry's answer (he presents it as an exception). Why would the poltergeist then assume that out of the blue they could not be visible to him? Wouldn't this imply that ghosts can use magic to hide (also from witches/wizards then, which would contradict the solid characterization above)?

And why should ghouls be invisible? They have a corporeal appearance and have no magic powers.

Of course, it could be that Peeves asks the question this way simply because he can't see anybody in the dark of the night but given Harry's answer and the fact that Peeves believes him, the issue as pertains to ghosts remains. I'm curious if there is a good explanation other than assuming Peeves is stupid or too scared of the Bloody Baron to question Harry's answer.

Answer 1

Ghosts can at the very least hide themselves out of sight.

There’s no canon information I’ve found about whether ghosts can actually become invisible. However, they certainly can hide. Moaning Myrtle was able to hide from the Prefects to spy on them unnoticed in the bathrooms. It’s unclear whether she made herself invisible to do this or if she just hid, but it doesn’t seem like anyone noticed her there.

“Have you been spying on him, too?’ said Harry indignantly. ‘What d’you do, sneak up here in the evenings to watch the Prefects take baths?’

‘Sometimes,’ said Myrtle, rather slyly, ‘but I’ve never come out to speak to anyone before.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 25 (The Egg and the Eye)}}

People usually avoided Myrtle, and would even more want to avoid her if they knew she’d be peeping at them in the bath. The girls’ bathroom Myrtle was hardly in use because of her presence, but the Prefects’ bathroom didn’t seem to have this same problem. We know she spied on Cedric, and Cedric wouldn’t have likely known this and let Harry use there without warning. He wouldn’t have noticed her presence, however it is that she’d hidden, so ghosts are provably capable of not being noticed. Peeves doesn’t seem to have any extrasensory perception, so he might not automatically know the difference between a well-hidden ghost and an invisible one.

Ghosts may be able to become invisible, but it’s not clear.

It seems at least possible that ghosts can become invisible. It’s never said explicitly that they can, but it is mentioned, however, that Peeves (and possibly poltergeists in general) can turn invisible.

“Nearly Headless Nick was always happy to point new Gryffindors in the right direction, but Peeves the poltergeist was worth two locked doors and a trick staircase if you met him when you were late for class. He would drop waste-paper baskets on your head, pull rugs from under your feet, pelt you with bits of chalk or sneak up behind you, invisible, grab your nose and screech, ‘GOT YOUR CONK!”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 8 (The Potions Master)}}

However, the J.K. Rowling Pottermore writing on Peeves says poltergeists are by nature invisible, and they certainly aren’t ghosts - so this isn’t proof ghosts can as well, but is notable as they’re somewhat similar.

Answer 2

Not sure what Peeves was thinking in that precise scene.

However, in PoA we see Harry in the invisibility cloak scaring Malfoy and his goons (after he sneaks into Hogsmeade) by hurling mud at him, who thinks it's possibly the invisible ghosts supposedly haunting the Shrieking Shack.

Although they live in a world full of magic where randomly flying objects should be an everyday occurrence, or, at least not something strange, the fact that Malfoy was scared it might be invisible ghosts suggests that ghosts or possibly other evil spirits becoming invisible is not something unheard of, in the Harry Potter Universe.

Thus it's not strange that Peeves believed the Baron had turned invisible.