Slight water leakage under toilet

I have an upstairs toilet that drains directly across to a pipe on the outside of my house, so all the water flows out horizontally. There is a little bit of water round the back of my toilet that appears to come from under the bit where the water sits (apologies for my lack of technical terms) the pipe that goes out and joins onto my wall has no leaks at all and there is no pipes under my toilet that could leak, what do you think could be the reason for this?