One of the major problems with the suggestion "The washing machine signaled off" is ambivalence - does "off" refer to the washing machine, in which case "signaled" doesn't fit - "turned off" or "switched itself off" fits better - or does it refer to the signal, in which case the implication is that the signal itself ends? If we want to keep the "The washing machine signaled" phrase, we have to say something that clearly is a signal, unambiguously, e.g. "the end of its cycle." However even that feels a bit cumbersome. The Oxford example comes closest to what an English speaker would say, in my opinion. Myself, I'd most likely say "The washing machine beeped and turned off," or something similar.

One of the major problems with the suggestion "The washing machine signaled off" is ambiguity: is it the washing machine that's now "off", in which case "signaled" doesn't fit ("turned off" or "switched itself off" fits better), or is it the signal that's now "off"? If we want to keep the "The washing machine signaled" phrase, we have to say something that clearly is a signal, unambiguously, e.g. "the end of its cycle." However even that feels a bit cumbersome. The Oxford example comes closest to what an English speaker would say, in my opinion. Myself, I'd most likely say "The washing machine beeped and turned off," or something similar.