Local guy has it installed on a 93 LT. Wants to get rid of it.  The horn sounds at inopportune times and it didn't seem to make a whit of difference when his bike got stolen last year.

I know it's a BMW alarm because the arming keychain fob has BMW on it and when I opened the relay box yesterday it was connected with the proper connector.

I was able to disable the horn going off by simply unplugging it in the relay box but he'd like to get rid of it altogether. It doesn't look like a hack install so I think it was installed by a dealer when the bike was purchased.

Anybody got any of the following docs for it?
1 - Install instructions
2 - User manual
3 - Wiring schematic

There are links to various alarm instructions and diagrams in this MB.C thread starting with post #7 and more in post #12.

I've followed the links here back as much as I could (some time has passed since this discussion and stuff has vanished). It appears that the BMW alarm system on my 1996 K1100LT is identical to others mentioned, but I think the harness must have been custom made.  This afternoon I opened the relay box and found a number of non-OEM connectors and some notes left by the installer that just mention "bypass 11/04".  
My questions is: if I simply remove all the connected wires that are obviously non-OEM, will that suffice for removing the alarm system?  My fear is that I'll inadvertently render the bike inoperative.  If I disconnect the plug at the control unit, this is exactly what happens....
In the course of examining the relay box, I found evidence of an overheated brown wire (that exits the relay box at the right rear side).  Aside from painting the wire jacket with liquid tape or some such substance, what should I do about this? What would cause this gnd. (?) wire to overheat?
I should add that all functions of the bike are currently working as they should.  However, this inoperative (no control fob) security system serves no purpose except for being a electrical problem waiting to happen.

You might get some help from this post on alarms.https://www.k100-forum.com/t701-wiring-diagram-for-alarm-unit
Regards Martin.

Alarm Schematic

The original connection of the alarm goes also on the temperature control unit. I recall reading somewhere that when the alarm is on, it ground the water temperature sensor to disable the fuel injection (sending a signal to the EFI computer that the engine is overheating).
I have somewhere a documentation about the alarm and will try to locate it.

IME your overheated brown wire is possibly caused by corrosion under the main (black) battery cable where it bolts to the gearbox. The next easiest return route for the starter is through the frame and that wire.

IME = exactly two bikes with the same symptoms

Just a little update on this whole alarm thing.  My unplugging the horn connection did accomplish the goal of keeping the horn from going off but he called me a couple of weeks later to report that his turn signals were doing funky stuff.  Not being used to dealing with alarms, my first instinct was to assume that his turn signal issues were due to a dirty starter.  But when he brought the bike by my place for me to take a look for some reason I remembered that he had the alarm installed and concluded that what the turn signals were doing was sending some sort of alarm-related message that one would need a manual to interpret.

Upon further inspection I found a BMW labeled black box in the tail cowl which could only be the control unit for the alarm.  I undid the connector to it and his turn signal functioning returned to normal.

You are right Duck
The alarm module is in tail.