This is very strange.

Did you have any front suspension work done recently (before the incident)at that wheel; for example, struts, CV joint, driveshaft (axle) or a bearing?
When the proper nut is used, and assembled properly, it cannot fall off by itself. One of three special retaining methods are generally used . . . a cotter pin, a staking tab, or a nut with integral thread locking. I think a 1998 V70 uses the last method. If the locking mechanism is damaged during disassembly, a new nut is supposed to be used.

If your V70 is a left-hand drive model, I would expect the normal rotation of the spindle to tend to tighten the nut over time, not loosen it. Perhaps I am not thinking about this right.

In addition, the spindle (axle) nut is covered by either a wheel cover or plastic center cover. If the nut came off, it should have rattled around inside. Even if the wheel cover were missing, it seems like a one-in-a-million chance that it would happen to fall off in your garage. Did your mechanic remark that this nut was missing? This would be very unusual, something he would remember to tell his grandchildren in old age.

I suspect one of two things . . . human intervention (perhaps the nut was loose and a child was able to spin it off), or that the nut you found in the garage is not the spindle nut.