strange occurance in regretevator (roblox), but how'd it happen?

This happened today, but I was playing the Regretevator on roblox, when something seemed to have the roblox studio player indicator in my private server somehow (and I was alone in the private server as well), their name appeared to be something along the lines of "WilliamPlaysS7" or "WilliamPlays57" with it spelled strickly so. but I tried searching their name with no perfect matchups. I'm wondering if a hacker joined or a owner managed to go in roblox studio while in-game somehow, but if you have any ideas, be sure to let me know.