Players getting kicked after joining w/ mods

Papa, we have noticed a few players logging into the server apparently with the mod downloads (they get in and move around) but then they are kicked shortly after by SC. Is that server connection or do we know what causes that? thx

Do you know these players? Or have you spoken to them?

If they are kicked by SC, that is almost certainly the anti-hack, unless they are getting some sort of other messages.

It may be attempted hackers.