Nothing guaranteed of course but I would have thought you'd be OK leaving a car for one night in Kinlochleven.  There are public car parks at NN188623 at the back of the village and NN187618 behind the Co-op.  There's also parking next to the Ice Factor but not sure if there's any restrictions.  To be honest you could probably park at the side of the road as well which might make your car less of a target.

You coud probably park at the Mamore Lodge Hotel. They let people park there during the Day for £2 , I would think they would let you park overnight as well. They are a nice bunch and they do the best hot choc in the bar for when you get back. Its a handy place to park as well if you are heading up the Mammores.

I've left my car in the carpark for the Grey Mares Tail waterfall on several occasions and never had any problems. However, Mamore Lodge does cut out a couple of hundred metres ascent.

I've left my car in the public car park behind the co-op and out side Blackwater Hostel and never had any problems.

Mamore lodge again, I've used it a few times.