I'd start by searching and acquiring a electrical schematic for your make/year/model bike. Or just pull the cluster and start physically following the wires back looking at every connection and making sure it's good. Might also just be the bulb in the cluster and it's socket being corroded. But I hate those kind of bugs.
This is where the wiring diagrams come in and could really help. I'm just guessing that somewhere on your dash you have signal light indicator lights. There may be a common connection (dash lights/signal lights) that's corroded or damaged somehow.
You could also "attack" this problem by trying to fix the signal lights. Fix that and the other problem is fixed also.