Good question and I don't know the answer. Most of my experience has been as a ground pounder, so we don't often interact with signals designed to attract attention from the air. I have occasionally ridden as an observer and as cargo enroute to a remote dropoff point. In that context, we would investigate any kind of signal that might be connected to our operation.

That doesn't say anything about the signal that might be in view of non-SAR aircraft however.

I'll bet AKSAR has a more definitive answer....

I am reminded of the Gary Larson cartoon showing a helo in flight and HELF spelled out on the ground, with the caption along the lines of "Sorry, I thought it said HELP - my mistake"

one of his very best...