I've been experiencing this issue way too often on my '93 K1100lt.
Start bike, I hear starter still cranking, I have to remove seat- lift gas tank up- lift cover up- and tap on starter relay box to get it to stop!
On one hand I heard this can happen more easily with a weak battery. But I had just rebuilt my alternator and I have a brand new battery!
Could I just need a new relay? Is it a starter relay or starter solenoid? Is there a radioshack or other cheaper part I car source besides BMW OEM?