The mooring (pontoon) of the speedboat is B3 (behind us), we are on B4.
I can not answer as to why someone locks up a boat at a mooring that is not theirs as I did not do that!
We asked the harbour master what to do when we found the speedboat at our mooring and he asked us to moor up on B3 (the speedboat's place).
Sure it can not be a mistake as when he saw my fixed mooring lines, he should have realized that something was wrong. The harbourmaster confirms that B4 is our mooring. We have been at B4 last year and this year since the boat was launched again and put in the right place (by the marina).
I just had a call from the harbourmaster that the speedboatguy brought a spare key and that the harbourmaster will put the boats in the right place this week.