There are two reasons.
Number one is when I'm asked to investigate something by a member of the House of Commons. I have no choice; I have to do that.
I look at it, but I actually do what we call an exam. We look at the situation and ask if there is enough. Is this just an allegation, a rumour or something, or is there something substantive enough to have reason to believe that there has been a breach of the act? If so, then we start an investigation. If we don't have reason to believe that there has been a breach, then we don't investigate.
If it's on my own initiative, the same test applies. If somebody writes to me or if I see something in the paper or on the news, etc., and it looks like this is suspicious, then we may take a preliminary look at it and see if there is enough. We'll ask for some documents from the relevant person. On that basis, we make the decision of yes, this warrants a deeper investigation, or no, there's really nothing here and this is a bit of vapour produced by the media.