Hi Ian....welcome to the Forum
It could be that a bulb has gone and it is using the next nearest (and most suitable) bulb as a substitute.
Got caught out on this with an MG a few years ago and only on close examination I noticed that is was the bulb below (the brake light bulb) that was illuminating as a side light to remain road legal. It cunningly uses the CanBus network to throw a voltage elsewhere so you really need to carefully check and examine each bulb in the event of this occurring.
In the case of the indicator though, check that the side repeaters are working okay as they are measured as part of the circuit
Let us know how you get on with it