I've wondered that. Thankfully I've not tested positive at work the first year and neither time the gf was sick. We get plenty of free rest kits still up here so we usually test at the sign of anything. I think that's how I've been able to avoid getting it with the gf as she has been religious about testing right away at the first sign of a tickle.

But still if neither of us are sick or showing symptoms we don't test so it could have come and gone without knowing.