Howdy,
I'm a little rusty on FANUCs. But I think, if your 387 Alarm carries no label with it, it may very well be a machine tool builders alarm. There are some alarm areas that FANUC reserves for alarms set by the machine tool builder.
You might want to check the Main't manuals from the machine builder for your machine. They are usually listed in there. Also, if this is an alarm programmed by the machine builder, it will be set via the ladder. Probably take some searching, but it's there
I don't know where you're located but FANUC (in US is called Fanuc America)support is pretty good. You might try them or GEFANUC. They can tell you more if you can't get support from the manufacture. At least they can tell you if it is their alarm or the MTB.
Ed