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Turn the key and no joy.
I had this happen with my old 740, a frustrating intermittent gremlin.
Then it would start up immediately with a screwdriver across the solenoid lugs for a second and it would work for minutes or months.
I tried everything including swapping the starter, but the problem remained.
It's obvious that the solenoid is not getting enough current to start via the normal key circuit.
Why it usually resets after shorting the solenoid lugs is the $50 question.
The solenoid may hang up requiring more current than the key circuit offers?
The interlock switches and ignition switch may limit current, intermittently. How to tell?
here's a good idea from: https://forums.pelicanparts.com/porsche-911-technical-forum/196077-how-do-solenoids-fail.html
To make sure it was not an ignition switch problem, I connected a test lead to the proper terminal at the starter, and placed a test light on the passenger seat. That way, I could watch the test light as I operated the ignition switch. The test light turned on, and the starter did not operate. Bingo, bad solenoid or starter. And again, the starter itself was fine. Frankly, starters just operate a few seconds at a time, then they get to rest. Alternators are continuous duty. So, starter motors can last a while. Solenoids are the problem at least 80% of the time that starters don't work. And as I say, a new Bosch solenoid is probably just around $30.
sounds like it would make sense to do the test light thing, to ensure that the problem is the solenoid rather than the ignition switch. (and interlock switches)
Finally the 850 has a service socket in the engine bay,
It is simply a wire to the starter solenoid with a molex connector. Find out if the S80 has this service socket. If not your mechanic could install a heavy wire to the starter solenoid. Then when the key fails, you leave the key in the run position and put +12V to the service socket to start the car.
If you can see the solenoid terminals just short them with a big screwdriver
to do the same thing. This is preferable to the time and expense of waiting for a tow.
Finally there is also the possibility of the alarm module causing this problem since it has a set of series contacts in the starter circuit.
To simplify:
The test light for the solenoid could be run from the service socket,
which be a good way to find out what's wrong. It doesn't start and the light comes on, you have a sticking solenoid (very possible)
If it doesn't start and no light comes on, it's the series switches or ignition switch.
Good Luck, Bill
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