At my previous company, I was caught in a mass layoff (best I can estimate is it was ~10 employees at a site with ~200? It's been 12 years and my memory is fuzzy). The company was known for annual layoffs of a handful of people, and the typical timeframe for it was approaching.
Around 4 or so weeks before, I started receiving unusual interest in projects I was working on, as in my 2nd line manager (boss's boss) dropping by to make various inquiries - how's it going, remember to write up some documentation/process, do you think you'll get to the XYZ portions). It was unusual since my supervisor and I didn't talk about that during our one-on-one meetings - a more natural flow would be for boss's boss to follow up on issues I raised with my boss, or for him/her to relay their questions through my boss, not for him/her to inquire directly.
Second oddity was that weekend or the next weekend. I stopped by the office for a few hours Sat morning, and saw another even higher level manager, maybe my 3rd line manager (can't remember who reported on up etc). But I was previously a direct report of the 3rd line manager so it wasn't weird to say hi and chat. They asked what I was doing there on a weekend and if anybody specifically asked me to come in. Well no... at the time I thought it was a little odd to ask that.
Third odd sign was I put in a vacation request for Fri and Mon two weeks in advance, and it got denied. With the reason "you are scheduled for XYZ class on that Fri", and oh, I hadn't heard that class was coming or I was added to it. It was weird because typically they'd send email and ask who is interested in XYZ, we're thinking of bringing in a class, of these days which is good, etc. To find out you are in a class next week by denying a vacation request? That was weird. I asked around various coworkers and nobody else was scheduled for that class or even knew it was coming. Big red flag.
Final sign was the Tue before, boss emailed and said I could have Fri off after all. No mention of Mon off too, and no mention of class details the Tue before the class.
I figured out the layoff day moved to Thu so I would get Fri off anyway and of course I would have Mon off too (boss slipped up not mentioning it).
And sure enough, I get in Thu, I hear rumors that layoffs are occurring. After checking email and seeing nothing, I went up to the sales area (errand to run) and saw a few employees in tears while packing up. I return and my boss is looking for me, asking where I was, and oh we need to have a meeting right now. 2nd line manager is there and someone else I didn't recognize (i.e. the HR rep). Suspicions confirmed as I looked in from the hallway.