Not a problem?????

Flying on FSPax in a Learjet 45. It was only a small flight so my tanks were only filled to 40%. When I started the flight the weight in the tanks were equal. However, 10 minutes into the flight it appeared that the left tank was decreasing quicker than the right. I declared an emergency etc and by the time I got on final approach to the redirected airport the left tank had nearly 300 pounds less fuel. I wasn't on fuel crossfeed or anything. So you would assume that I had a fuel leak in the left tank but when I checked the report after it said I had declared an emergency without good reason.
Therefore, how am I meant to know when the times comes to having a real fuel leak. Why did this happen in the first place?
Od