The movies were right about this one: when people cheat, it's statistically far more likely to be a co-worker. If you consider how few people she'd be meeting in her day-to-day life outside of work, it makes sense. Not only that, but there's all those juicy opportunities. Business trips, office parties, tight project deadlines - all of these can turn intimate in the blink of an eye.
A regular office girl is bound to have these kinds of events every now and again, so if she's late home a couple of times a year, there's nothing to worry about. But if office functions and "lots of work" seems to come up with increasing frequency, you'd better check out her cover story quick smart with a couple of her colleagues. You might not like what you find.