WHen you right click the mirc icon in the system tray it displays the Networks and current Nickname with a checkmark when you uncheck the Network/Nickname you cannot put the check mark back by clicking on the item again. In mIRC 7.75

Thanks for your bug report. This is the intended behaviour. The checkmark indicates that there are unread messages in the window. When you select the menu item, the window is displayed, and it gets unchecked.