Sorry about the setback, OR.
For me, my knee thing is officially weird. I have some theories.
It was not a problem for Turkey Trot or the few days before. I took 4 days off after that to put it fully to bed. I ran on the 'mill on Monday to get back into the swing of things. I went slow and with no time rush ran probably 7.5 miles or so. Felt easy and great. Almost as soon as I get off of it, knee starts getting bad as it can be (awkwardly going down stairs, trying to shake it out etc.). Tried to run yesterday and made it about a half mile, before I succumbed to the discomfort and had a walk-of-shame home. Yet today, everything feels brand new. Should note in the early days of the injury I had constant minor discomfort especially on stairs or if I sat down for too long. That is not the case any more at least after a day off or good night of sleep.
Theory: Exercise beyond ~45 minutes triggers the knee problems. That could explain why it didn't come back in the week before the Turkey Trot for the most part, and if it did it was very late on in the runs. I went beyond that Monday and suddenly issues.
Tentative Plan: 2 weeks off, and come back slowly. I'll schedule a Dr./PT appointment as necessary if issues remain as I build up in run length.
Don't know if anyone has experience with an issue like this. I'm happy it fades aways so quickly, but if I want to get serious about my next goal (sub 1:21 in half next spring) I need to do lots of runs over 45 minutes and not worry about forced days off.