Thanks to everyone for your well-wishes. It seems I don't spread enough to hit you back which is an odd state of affairs.
I have to reckon that I sprained my knee somehow. I do recall twisting my knee to make it pop while I was out walking one of the dogs but who knows? My knees always pop and crack while I'm doing my initial three sets of squats, and often beyond, so I thought nothing of it at the time.
The knee is still stiff and it's still uncomfortable to stress it and it hurts slightly when I do heavy lifting, like picking up a 25Kg dog. As such, I'm still staying away from squatting. I had considered doing some wide and high box squats but am leery of worsening the knee. I think that until I can do a free-standing squat without discomfort then squats are right out as are deadlifts or anything similar. I'm not even happy about the idea of doing OHP for fear of over-tensing the joint and hurting it. All it would take would be a slight topple and I could lose control of my balance.
On the other hand, it does feel slightly improved pretty much every day.