Logs are optional. If you have them enabled in mIRC's options (Alt-O > IRC > Logging), then the logs are saved in the location you have set up there. If logging is disabled, no logs are saved.*
You can quickly find the log folder by typing the following into your edit box and hitting Enter:
//echo -a $logdir
Or, to open the folder, type:
* The Alt-O options are default settings and can be overruled by the logging options set by right clicking on a channel or query window. So even if you have logging on or off in Alt-O, you might have it set to the opposite for a specific window. You can right click that window and choose Logging to change the settings for that window.