The thing that tips off a woman is any perceived change in behavior. If you are usually inattentive to her at home, and suddenly start to shower her with gifts and attention, she will be suspicious. Its not always the physical clues like perfume, lipstick, bank receipts, or late nights at the office.
Fortunately, I can hobby in relative safety since I'm off on "business trips" when I come to Toronto, and I do not partake in my home town under any circumstances.
However, when I did, I simply made sure to do it at time when it did not conflict with my normal life. Lunch hour appointments, Saturday daytime when I was "running errands", and I never ever left a paper trail of information to find.
You might also consider the safety of any e-mail correspondence, particularly if you are careless about clearing passwords and cookies on your computer.