It seems that there is something wrong with the Party Invitation in the Boyle party mission. The pointer marks the ground as the location, but there is nothing there. It seems that the wind should blow it there but as soon as it lands it disappears.
Jesse Cox also got this problem in his video "Dishonored [Part 28]: Distance Apart but Never at Heart" although he doesn't seem to realise that. /watch?v=lQOcZv2C2o4 (the forum doesn't let me to link it)