Talk of the office
Everyone knows news travels fast in an office, and you can pick up most things that are happening around the coffee machine. Don't harbour any illusions that you'll fool people: someone will have spotted you and the dishy new trainee sneaking off to the back stairwell, or spied you snogging the boss in the car park. And once tongues start wagging, it's not long before everyone knows exactly what you've been getting up to!
If you're cool with it and strong enough not to let gossip get to you, great. But prepare for the worst. Not only will news of your liaison go around the entire company: it'll also be taken to pieces, exaggerated and twisted. Aside from the fact that it could affect your professional reputation, it's not much fun when everyone in the office is talking freely about your sexual prowess! People might stick the knife in and say you're trying to win a promotion on your back or that you're sleeping your way through the company. It's not fair, but it happens. Just tell yourself they'll find something else to gossip about in a few weeks!