Actually, the government is looking into creating a new class for intellectual properties called, "Orphaned Works." The gist is, if you don't know who (if anyone) rightfully owns a work, and you've made every effort to find out, you place some kind of symbol or something on the work in recognition of the fact that it is an "orphaned work." It means, that you may legally publish the work, until such time as a rightful owner of the work comes forward. It is protection against a lawsuit. I have been very interested in this, because my website (see my sig), is an effort to collect information on usable characters, and I'm thinking about adding characters from a 1980's action figure line (with mini comics) called "Eagle Force" which was produced by a company called MEGO. Mego is now completely disbanded and the rights to Eagle Force, as far as I know, were never sold to anyone. At least, nobody has used the characters in 30 years. They have been orphaned, and I would love to give them a new home. But I admit, I'm a little afraid to tell people they can use these characters, until I have contacted a known artist who was about 80 percent responsible for creating the characters. I imagine the same dilemma exists with fly-by-night comic companies. Many of those were creator owned, so their creations will be owned by them until well after their deaths. However, if there is evidence that a now defunct comic company hired the creators, and legally owned the character copyrights, then those works would be considered PD or at least, orphaned. As far as posting comics online, you can probably get away with posting any comic if it is not making anyone any money, regardless of the law. But, if you plan to make money off of something, you need to do your research.
I am actually in favor of companies like Marvel maintaining the copyright to works like Thor which they actively use. I just wish the corporate giants weren't so stingy about all their long forgotten characters. People say, "well, why not just invent new characters?" The invention of new characters is great, but my problem is that a lot of "new characters" are just rehashes of the old ones. There is something to be said about the familiarity of an old character, and demonstrating some pride in that character's heritage, while creating something new and amazing. You can take Dracula or King Arthur and run with it a million different ways. Let Marvel have their Thor if they use him. There are still plenty of specific characters, including alternate versions of Thor, which us creative types can use. The key is to give them the kind of recognition that Marvel's version has. The difference between Marvel and Kirby, isn't the characters, but the support given to the characters. With sites like mine, and enough people supporting them, we can make PD characters just as recognizable as anything in the Marvel stable. Some of the PD superheroes are even better than some of the ones Marvel has going for it. Let them have their crazy Shakespeare talking Thor, if we can generate a PD version with even more presence.