If its gonna be somewhat obvious, sometimes better to tell the truth to people who ask...but MOST would be too afraid to ask anyways. Yet in this day and age with the internet, a lot of males have done at least some research on hairloss/HT, so its hard to trick everyone.
I work in an office environment and had a coworker who had a obvious transplant (frontal recipient area was shaved, back wasn't to probably cover the scar). He told people he had an accident and needed head surgery. During drinks after work, when everyone gossips like crazy, a lady co-worker blurted out to others this guy for sure had a HT since her husband was also interested in one and had been researching the internet. Next thing you know, a few people where sending emails poking fun behind this guy's back. Kinda felt bad for him, but at least he had no clue about it.
Its always hard to predict how much people know and whether they'll gossip/make fun behind your back...especially in a bigger office.
I totally envy people who get to wear a hat to work, it makes a HT sooo simple.