The first time many years ago a few weeks or less after I washed the engine, for the first and last time.
I recall the light was on and the spoiler was up when I pulled in to the parking lot here at the office.
I tried to lower the spoiler with the switch but it was not moving. I left the car parked and later came out and drove it to lunch and back. At some point I noticed the light was out and the spoiler had retracted I guess during the drive to/from lunch. (Oh, driven on surface streets at speeds far below the deployment speed.)
Another time more recently but still months ago (I think I posted a picture of the spoiler light on, the CEL on and some other warning light on) it came on for no reason but I do not recall the details. It went off as inexplicably as it came on.
The spoiler continues to AFAIK deploy when it is supposed to and retract when it is supposed to. After I got the Boxster back from the accident repair I tested the spoiler both on the highway and manually. I checked the spoiler doesn't have any excessive play and does not show any signs of any wear/tear.
I suspect what is going on in your car's case is what I suspect went in my car's case, both times. Marginal electrical contact at a connector at the spoiler raising/lowering mechanism or a switch. If you want to track down the spoiler wiring harness and disconnect and connect and related connectors with the idea this will remove any surface corrosion that may be affecting the signals to the spoiler or from the switches.
You might also remove and re-install the spoiler relays. I think there are two, but I may have my 996 Turbo's spoiler confused with my Boxster's spoiler.
You can also do what I did: Ignore it with the idea if the light doesn't come back on and the spoiler continues to work there is no problem. So far this has proved to be the case with the Boxster
In the case of the 996 I had to replace the spoiler hardware, at a considerable cost when the warning light on symptom was soon joined by signs of fluid leakage at the spoiler cylinders (the spoiler system in my 996 is hydraulic) and with uneven spoiler deployment.