>> False positives do occur on occasion
Some "False positives" that show up on some peoples PCs are either key-generators or program cracks, because technically cracks are a trojan in-so-far as they are designed to change part of a programs normal operating process without the programs permission.
>> where would I look for dodgy files/how do you tell?.
By running at different times using different programs then placing the result/s and search on the internet (or Googling as some may say) to see what other people are saying about them.
Some time back I tried three different programs one after the other and all three came back with a degree of different results.
So what one program may pick up, another will either not know about or ignore it as being safe.