First of all: I checked all of options here: mirc options / Display / Options / Tray / ..
After I leave the computer for hours (eg: 10-20 hours) the mirc icon disappears from tray but mirc still works.
Mirc is always runing on my htpc, it start with windows (a shortcut in win 10 startup dir).
I have no possibilities to get mirc's gui so I can only stop mirc's process with task manager.