No, there can never be more then 3 boats in a neighborhood.
Thematic: the thief has to be in the neighborhood before he can steal anything, right?
Correct, on no more than 3 boats in a market hood (this is the term used in the English section of the rules.
The thief placed like any other boat according to the Rules of movement. This may have it in a market hood with other boats or by itself. Only after placement is another boat stolen along the horizontal line (see the arrows on the boat), regardless of distance. An inspector's boat may not be taken.