I'm kinda sure it's okay to park there at any time, but you may save yourself a deductible by rolling the windows down and removing the radio.
Parking at the Greenfield Ave Eliza Furnace Trailhead at Night
Is it OK to park in the parking lot at night?
I go through that parking lot quite frequently at at night (11pm-2am), and there is always a few cars down there.
Officially, parking is only supposed to be there dawn to dusk but that would go against the other signs that say it is for the use of trail commuters when winter comes around and the days get short.
Mine is one of the cars that is often there into the night. Once a week till about 11pm. I have never had a problem from either miscreants or the constabulary.
That is not to say there are no problems. There have been a number of break-ins over the years. Most of them seem to be during the day on weekends. You take your chances but I'm not sure it's any better or worse than anywhere else.