I doubt it. If it was a bare lamp against the grass it could happen but the only way there's a big enough chance is in the winter or early spring when the grass is dried out. Since the grass has moisture in it, the chances of a fire are small since the lamp in is a fixture. I'd snag the PC though just to keep the light from coming on since someone could get hurt if they tamper with it and aren't educated with electricity.
If that happened here a fire would start quick, it's so dry here. There are forest fires raging all over the place. I think they might be getting more contained now though...
Well the lamp is mounted on the top housing and is not touching the grass....plus the sidings are plastic and it's only 70 watt....nope ain't gonna be starting a fire, I saw it again, was still like this....it has a Tork brand photocell
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but like you say, its very dry. wouldn't take much for another to start...