Where did you open / pitch the yeast? Did you sanitize the hydrometer / flask before pouring in the wort?
I've found that yeast are pretty darn tricky... I mean, come on, they've been around a billion years or so, right?! They can go dormant and "live" a LONG time in cracks, crevices, and even airborne. So, what MAY have happened is that some yeast from a previous sample was left in the tube or on the hydrometer and when it was placed in the nice, warm wort in the nice, warm garage, it decided that it would just live there and do its thing. Or, it could have even come from when you opened the yeast.
Or, it may be that you've picked up a yeast from your garage. That is a little less likely but entirely possible. It doesn't take 2 weeks to get Sacc, it could take as little as a few minutes if it is floating by and settles into your wort.
Don't know if that helps at all but that's my take.