kids loitering in gangway

[Nicko <nervous.nick@xxxxxxxxx>]

I have recently had problems with teens hanging out smoking weed in
the gangway immediately outside the back door to my apartment.  There
is a breezway along one side of my 22-unit building which is open
except for the section by my back door, which lies in a covered
section of the gangway between two gangway doors.

I have talked to the kids politely, to no effect.  I also called the
police, equally in vain The kids must have been watching out the
gangway door and saw the cops coming, and ran back into the gangway,
up the back porch stairs and into the apartment where one of them
lives.  The police walked up those back porch stairs to check it out,
but of course found nobody there and could do nothing, legally.

One of these kids lives in the building, and they are gangbangers,
most likely, so probably talking to the parents who live there would
not be a good idea if I want to stay here, and I do want to stay
here.  I have no problem with kids smoking weed, as I did enough of it
in my youth, but it is just WRONG for them to be doing it within feet
of my door--especially if my girlfriend is home alone.  And who knows
what silly ideas they might get about breaking in when we are not home
(though the door itself is alarmed for entry).

Yesterday I heard the kids out there smoking dope again, and I got
what I thought was a good idea:  I took a smoke alarm and held it up
to the door and pressed the test button.  The alarm went off, and the
kids immediately vacated the gangway area outside my door.  I
laughed!  I hope it scared the fuck out of them, but given that they
are teens, it's likely they didn't get the message--not to mention
that they MUST have known the police came up there looking for them
that one time.

Is there some kind of proximity or "anti-loitering" detector that I
could mount inside my apartment/on the inside of this door, that would
chirp a couple of times when someone passes through the gangway and
past my door, and only activate if someone stays in the area outside
my door for a defined amount of time?  And then (important when we are
not home) shut off after a set amount of time?  Is this too much to
ask?  Probably.

Such a device would have to "see through" the door, most likely by
induction, but I am open to any ideas.  The door is metal, btw, which
I know might be an issue, but I could probably rig up something
ungrounded between the gangway and the inside of my apartment.
Placing a device in the gangway itself is not an option, because of
the landlord and because the kids would just rip it off of the ceiling
or wall.  The enclosed area of the gangway outside my door measures
about 15x7x5 feet so the kids are never more than, say, six feet from
the door.

Thanks for any replies to this.  I apologize for Googleposting but I
have my excuses.

Cheers!

Nicko