UpTimeRobot is a service people use to monitor websites via HTTP. No one requires anyone else’s permission to monitor a site via HTTP; however, I’ve never noticed unusual spikes in traffic by them, either, so you might want to verify it’s actually them?
In any case, if so, that just means that someone is monitoring when and if your site is down.
Edit: Here’s a list of their monitoring locations and the IPs for those locations, to verify for yourself