well i know that warning blasts do happen when a ship, such as titanic and others are due for departure...
eg.
PS Waverley : warning blast 20 minutes before departure from rothesay (isle of bute, scotland) peir (normally only on fridays when passengers get an hour ashore)
MV Balmoral : warning blast 10 minutes before departure from Glasgow's science centre complex (since the broomilaw has been cut off due to the new Clyde Arc bridge (aka "the squinty bridge") only heard it once when she was going on passage to greenock, helensburgh, dunoon, largs & Milport, for the illuminations on 22 september last year.