Most of my guinea pigs have never sensed when I'm feeling low other than one.
My boar Romano was recently by himself as his cage mate had to be removed after a falling out. I was looking to get him a friend but for now while he healed he was alone.
I was really depressed over having no friends and as I cleaned out my other two piggies - who were out playing in the hallway- Romano came up to my bars and placed his two feet on them, leaning up to me as though to check I was okay.
He was a very skittish guinea pig at that time, terrified of me so when I saw him come over as I was upset and reach towards me, I think that he definitely knew something was wrong. He'd never approached me willingly before.
He was always a very affectionate boy after that and even in his final moments, he jumped out of his pet carrier (we were at the vets) to climb on my lap and he curled up resting his head on my belly.