Yeah, and it's not just "safe," it's a matter of being right, since every book that slips through the cracks here makes us seem a less reliable source. Something isn't in the public domain just because we might not get caught.
It's one thing if the properties were sold to an entity that no longer exists, but if Dell still owns the properties, it's entirely in their rights to do nothing with them and pretend they don't exist. "Abandonware" is exactly the idea that got the Blackmask guy into trouble, remember. He decided that the Doc Savage stories MUST be in the public domain because Conde Nast hadn't used them and didn't acknowledge them.