I'm pretty much an expert on chirping haha.
I've heard chirping 5 times, and i haven't even had my guinea pigs a year. The first 3 times it was a normal night, (well, it was evening, but whatever) and i was sitting downstairs with my parents when all of the sudden i heard chirping. I was thrilled. We heard it 3 times that night. After the 3rd time, my dad decided that there must be some sort of dangerous crisis upstairs, and demanded that i bring Pepper (the one we suspected of chirping) downstairs. She was fine, btw. Just a bit confused, since she had never been in the living room before.
The second (well, fourth) time was the day that Shadow (my other piggie) died. It was 3:00am, and Pepper just started chirping. It wasn't a good chirp though. The whole thing just didn't feel right. Grim, i suppose you could say. Shadow died the next day, but when i woke up i found her lying on her side in a corner, barely alive. I think Pepper was saying goodbye. Pepper absolutely adored her.
The third time was just a normal day, it was 1:00am when i heard her. I tried to see, but she stopped whenever she saw me move. There was no reason for her to feel any different than she would on any other day.
I also think Simi, the piggie i got after Shadow died, has tried Chirping but can't do it. She always does it when there are birds chirping outside. It sounds like a broken chirp, like her voice isn't suited for it. It doesn't sound good eitherl. Like, it doesnt sound healthy. But she's been to the vet, and she's in perfect health.