What a strange find

So today (as it wasn't raining for a change) I spent the whole day in the garden. I prepared all the borders and put out the bedding plants, so now all ready for summer. I was pulling up an old plant which was spilling over the wall, when I noticed something round just sitting on top of the wall. I was going to throw it away but on closer look it turned out to be a coin.

I have looked on the internet (whatever you have got someone is selling one on ebay) and it turns out to be a French coin dating from the 1790's, minted in Paris - not a very good photo, it is much clearer in real life. On the back it says Cinq Centimes L'An .... (unfortunately the year is obliterated but it seems the number usually varies from 5 to 7 ish).

So how did it end up in my garden, and what was it doing just sitting on the wall? Presumably it was pushed up by a plant and left for me to find. I wonder who was the last person to hold it? Could it have been dropped by someone at the private school which was on this site many years ago?

How strange.