Oil filling up problem

When I got my CBR250R two years ago, the oil level started to fill up for some strange reason. I took it to the dealership where I bought it who told me it was because I put too much oil on the chain after washing and that crept into the oil.

14 months later, the same problem arose and the dealership said that I had over filled the oil level - I never have and still haven't ever put oil in it. We agreed to disagree and I had to pay to get it repaired.

Now, after leaving my bike unused for some time, the oil level which was going down, is now over the full mark!

Any ideas? Petrol or break fluid seeping into the oil?