I was the safety officer at a boat company for a while, so I think I can chime in. We had several spontaneous combustion incidents like this. It's not uncommon. Acetone and solvent rags SHOULD NOT be tossed in your garbage can if they are not completely dry. To be extra safe in your shop, get a separate metal can with a lid and keep it away from your grinder. Paint cans work good for this.
We almost lost the company on a few occasions when teak oil soaked rags spontaneously combusted. In both cases they were mildly saturated with teak oil, wadded up in a ball and left on someone's work bench during lunch break. About 30 minutes later they started to smoke. In one instance, the entire wall caught on fire, and another caused a dumpster to catch aflame. I can't help but think there may have been acetone involved as well, but in both cases the investigation revealed that there was only teak oil on the rags.
It's even more important to put buckets and large cups of recently hardened epoxy outside and away from other combustibles, or you can just spread it out on a cardboard box so it's thin and can't build up heat while it dries. I forgot how many near fires we had because gallon size paper buckets of resin built up heat inside. Buckets of resin are especially dangerous because the outside gets hard while the inside becomes extremely hot, eventually the mass of hardened resin cracks, causing the heat to escape rapidly and start a fire. The buckets would get so hot that it was like cooling off hot steel when we dropped them in water. The danger increases when you use too much hardener and it reacts very quickly.
These issues were, without a doubt, the hardest to keep people from doing. Regardless of how many people got into trouble, it just kept happening. Its so easy to just toss stuff in the trash, but when chemicals are involved it becomes pretty dangerous, even when it seems harmless. Having said that, I still get lazy and break my own rules when it comes to chemical rags in the shop garbage. However I never mix too much resin, because I'm a miser.
Take care,
Brook