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In Chapter Fifteen of Philosopher's Stone, when everyone gets caught out of bed on the night Norbert was given to Charlie's friends, Professor McGonagall says the following in her rebuke:

"Four students out of bed in one night! I've never heard of such a thing before! You, Miss Granger, I thought you had more sense. As for you, Mr. Potter, I thought Gryffindor meant more to you than this. All three of you will receive detentions — yes, you too, Mr. Longbottom, nothing gives you the right to walk around school at night, especially these days, it's very dangerous — and fifty points will be taken from Gryffindor."

I am particularly interested in the phrase "especially these days, it's very dangerous". I am wondering why McGonagall considered "these days" to be particularly dangerous. These days were quite possibly the least dangerous in the entire series. Consider that the next year there was a giant monster on the loose attacking students; the year after that there was an escaped mass-murderer (supposedly) coming after one of the students, and he even breached the castle's defenses on multiple occasions; the year after that there were odd happenings throughout the year, culminating in the return of the most feared Dark Wizard; over the next three years the aforementioned Dark Wizard was steadily gaining power and killing people indiscriminately.

By contrast, in Philosopher's Stone it was a full ten years since the Dark Wizard had been heard from, there were no strange occurrences, there were no indiscriminate deaths, and there were no giant monsters or escaped murderers on the loose. The only thing that was potentially amiss was that an attempt had been made to steal the philosopher's stone a few months earlier.

Of course, McGonagall wouldn't have known at that point that the upcoming years would be so much worse, but even compared to previous years it doesn't seem like there was anything particularly dangerous about the times in Philosopher's Stone.

So why exactly did McGonagall think that these were such dangerous times? And she apparently expected the students (and the readers) to understand this, so we would expect there to be something obviously dangerous already introduced at that point in the story. (I.e. the answer should not really be that she was privy to some secret information from Dumbledore regarding suspicions about Voldemort, or the like.)

Notice she doesn't say it's dangerous to roam the dormitories at night, she says it's dangerous to walk around school at night. Admittedly this can mean walking inside the school, but it could be that upon using the phrasing, she added "these days" upon thinking of an accident we know happened, well, around the school.

When Harry, Neville, etc go on detention in the Forbidden Forest for that escapade, Hagrid states:

“That’s unicorn blood. There’s a unicorn in there bin hurt badly by summat. This is the second time in a week. I found one dead last Wednesday. We’re gonna try an’ find the poor thing. We might have ter put it out of its misery.”

_{^{Harry Potter and the Philosopher's Stone, chapter 15, "The Forbidden Forest"}}

As discussed in chat, the timeline is a bit unclear: after they get caught, we have "the next day" and then "Then, about a week before the exams were due to start" and then "The following morning"; the detention happens on that night. So there were at least two days between the incident and the detention, and possibly a bunch more, depending on what that middle quote means. ¹

If we admit the timeline went the following way:

1. Wednesday - first unicorn is killed


2. Saturday - Norbert is given to Charlie's friends


3. not long after, in the following week - Forbidden Forest detention

Then Hagrid could have told the other staff about the dead unicorn, which could have made Dumbledore think something iffy was going on, thus advising the teachers to stay on guard, leading to McGonagall's comment.

Admittedly neither the students nor the readers knew for certain about this, but:

• the trio being friends with Hagrid is no secret and McGonagall could have thought Hagrid told them too;


• the readers don't know anything yet, but are given the "unicorn" hint a bit later in the same book. That is to say, not just "they were both part of the Order and knew Voldemort would return, as they explain in book 5/6/7 blahblahblah". (although obviously the HP books have lots of foreshadowing)
  

Thought experiment: if dogs/cats were reported dead around a Muggle school, I'd expect the teachers to drop a line to the students about being extra careful. It might just be a coincidence, but it might also be a weirdo aiming at "weak" targets. Children may qualify.

¹_{Yes, I am using the OP's messages to answer their own question, and I'm not even ashamed.}