Once again, the source term is ambiguous, and only more intimate knowledge of the actual equipment will enable you to decide.

If it is indeed some kind of indicator light, I'd be inclined to say 'warning', as long as that is indeed the function — usually, a warning light comes on to indicate for example, that the oil level ,is low. However, it is by no means impossible that on this specific equipment, there is an indicator light to say that all is well.

However, a 'voyant' can also be some kind of sight glass, the sort of thing often found on larger installations.