Someone messed up and it could be kinda funny in hindsight that they put someone on your spot out of all the other possibilities but maybe they have an order in which they fill sites.
That being said, it is wrong. Wasn't the end of the world obviously but it does show poor management and I probably would have turned to angry myself if a front desk clerk smirked at me.
Also, a few people said you should tell the front office what you are doing if you leave for the day and I have to say I think that is absolutely ridiculous. If you paid for a site, you paid for it from check-in the first day through checkout the last day. If I don't want to even show up, I would fully expect a site to be sitting completely empty the whole time. You are paying them for the site, you shouldn't have to do any of their job for them.