Muggles from the wizarding perspective are seen to be quite inept at noticing things. So something like a vanishing glass because of magic would be common for them to just miss out, in the same way a bus roaring down the streets is just ignored by muggles.
"How come the Muggles don't hear the bus?" said Harry.
"Them!" said Stan contemptuously. "Don' listen properly, do they? Don'
look properly either. Never notice nuffink, they don'." - PoA
Furthermore, if anyone did notice anything, wizards put it on the fact that muggles consider magic impossible, and thus place it on the security that they will never blame magic or else be considered insane.
Still chortling, Fudge had thrown some powder into the fireplace,
stepped into the emerald flames, and vanished with a whooshing sound.
The Prime Minister had stood there, quite motionless, and realized
that he would never, as long as he lived, dare mention this encounter
to a living soul, for who in the wide world would believe him? -HBP
Not only that, but the ministry of magic has wizards on standby (like you said) to work their own magic to reverse magical damage and adjust the memories of the witnesses (if someone heard about an incident, but the main witnesses have no idea what they are talking about anymore...who would you be more inclined to believe? The witness who was there or the person who heard about it?).
They probably would have investigated more, but had their memories reversed. Note below, Harry’s Aunt, Uncle and Cousin didn't have their memory changed, as the Ministry knew they wouldn't blab to anyone about the magical incident.
"Eat, Harry, you look dead on your feet. Now then... You will be
pleased to hear that we have dealt with the unfortunate blowingup of
Miss Marjorie Dursley. Two members of the Accidental Magic Reversal
Department were dispatched to Privet Drive a few hours ago. Miss
Dursley has been punctured and her memory has been modified. She has
no recollection of the incident at all. So that's that, and no harm
done." -PoA
Another thing is Harry was still being monitored since his childhood, so his actions could have been reversed by Dumbledore and the remnants of the Order without his knowledge, or his Aunt, Uncle, or Cousin knowing.
"Why didn’t you tell me you’re a Squib, Mrs. Figg?" asked Harry,
panting with the effort to keep walking. "All those times I came round
your house - why didn’t you say anything?" "Dumbledore’s orders. I was
to keep an eye on you but not say anything, you were too young.
EDIT: It's also worth noting that muggles do notice these things, and strange activity, but they just put it down to being unable to explain bits.
Experts are unable to explain why the owls have suddenly changed their
sleeping pattern." The newscaster allowed himself a grin....
..."Well, Ted," said the weatherman, "I don't know about that, but it's
not only the owls that have been acting oddly today. Viewers as far
apart as Kent, Yorkshire, and Dundee have been phoning in to tell me
that instead of the rain I promised yesterday, they've had a downpour
of shooting stars! Perhaps people have been celebrating Bonfire Night
early -- it's not until next week, folks! But I can promise a wet
night tonight."
Mr. Dursley sat frozen in his armchair. Shooting stars all over
Britain? Owls flying by daylight? Mysterious people in cloaks all over
the place? And a whisper, a whisper about the Potters... -PS
Only Mr. Dursley who knows for fact about the existence of magic (despite being a mean old man, but smart and deft enough to run his own company) can start to add 2 and 2 together, but even then, he is skeptical about where his thoughts are going, and tries to ignore them.
