I've only ever seen commands beginning with "@" to be something you type into a #channel which interacts with a script that someone *else* has which looks for someone typing a message beginning with that keyword. So can you confirm whether this is really a #channel message that begins with that keyword? If it's a command to call your own alias, you would be instead typing beginning with a forward slash, /NameOfCommand
By default, mIRC logs all channels, and even the status window, though you may have turned that off if the log files got too large. The location where mirc.ini is located possibly has a "log" subfolder containing log files which might show you typing such a message into a #channel window.