How do I signal to passers-by that I am fine and just taking a photograph?

There is no established signal, which is a genuine shame — other activities have one. Divers have a recognised flag meaning there is somebody in the water, and the whole point is that it is understood without explanation.

Photography has nothing equivalent, so the practical answers are all improvised. The ones that work in ascending order of effort:

Be visibly equipped. A tripod standing beside her, a camera bag open, a lens cap on the ground. People read a scene, and equipment scattered around a prone person changes the reading from emergency to activity almost instantly. A tripod is the single most effective object because it is unmistakable at distance.

High-visibility clothing. Counter-intuitive for landscape work, but a bright vest reads as "working" rather than "collapsed" from a long way off.

Move occasionally. A small deliberate movement every so often is enough to stop most people approaching.

The other half is worth saying: let them come, and be pleased about it.

Somebody crossing a field to check on a motionless stranger is doing something good. Being briefly interrupted by that is a small price, and the alternative — a world where nobody checks — is much worse.

What I do is acknowledge early. As soon as I notice somebody looking, I raise a hand or call out that I am fine and taking pictures. That resolves it at fifty metres instead of at two, costs nothing, and usually gets a laugh.

A large proportion of the people who come over turn out to be interested in what you are photographing, and some of the best conversations I have had in the field started exactly this way.

The suggestion I have seen work best is printed clothing — a jacket or shirt with text on the back.

Something like "I am fine — photographing, not collapsed" answers the question before anyone has to ask it, and it is readable from the distance at which people decide whether to come over. It reads as a joke, it works completely seriously, and it makes people smile instead of worry.

It also has the advantage of working from behind, which is the direction people usually approach from when someone is lying face down.

A custom garment is not expensive and, as a gift, it is a rather good one for somebody who has this problem repeatedly.

One safety point in the other direction, since the concern is people worrying unnecessarily: it is worth telling somebody where she is going and roughly when she will be back.

Lying still in remote places, often at dawn or dusk, alone, is a genuine risk profile even though the specific alarm in this thread is a false one. The nice irony is that the same signals that stop strangers worrying — visible equipment, bright clothing — also make her easier to find if anything ever does go wrong.