Just my minor experience with self-seeding situations... albeit not as much of a mystery as your setup,
Is it possible the plant has gone through any other stress that the others havent?
Heres to hoping for sin semilla.
Everything that has been done stress wize is in my journal and applies to all 4 ladies.
But if I were to list them in order I'd say
Veg training
1.) First topping (along with branch manipulation for canopy management)
2.) Second topping (with canopy management^)
3.) Defoliating lower leaves and branches during mainlining training.
Once trained I flipped to flower.
No stress for first 3 weeks until I defoliated leaves off all 4 plants top %50 of branches.
Then at the last day of week 6 I defoliated the remaining lower half of all the ladies. Most of the leaves were very yellow and dying/wilting. They all pulled off easily with a light tug.
Watering/feeding consists of.
I feed nutrients until the soil is drenched but with minimal runoff (keep it close to 0 runoff). After watering I use the pot weight to determine when they need more usually it's 2-3 days later. But then I rotate between feed/water/feed/water etc.
I aerate my nutrients for a day or 2 before feeding so the microbes populate the jug & make things more bio available. While dissolved oxygen raises the ph closer to neutral and keeps anaerobic bacteria alive.
I also stay on the lower side of any feeding charts.
All 4 plants have been on diminishing light schedule since seedlings through the end of flower. But my photoperiod changes are all small (15mins a week in flower) and applied over a long period of time.
I'm sure there's a chance maybe a nanners never fully emerged from the flower. But I've kept a watchful eye on them throughout the whole cycle. Never to see a thing.
If there were any nanners or pollen sacs. Wouldn't it have pollinated every other plant too? I mean the clip on fan is pointed right In front of the plant in question.
Could the clip on fan have stressed the plant? It is positioned above the canopy line blowing towards the tent center keeping the canopy cool from the light created off the cooltube.
Here is a closeup shot of the SDXQ #1. It was already posted above^ but I noticed it has a potential seed in the picture.