This could be a highlight rule in the addressbook/highlights which has the option checked to display this. Otherwise, it could be a script having an ON TEXT event containing the $tip command.
You've got a file somewhere that has "new post" and "has posted in" in it. If it's a highlight rule, those strings are in the [highlight] section of mirc.ini, and if it's in a TEXT event, it's most likely in one of the scripts loaded in your alt+R window, though it's possible a script can keep such strings in some kind of settings file.