It's correct that the ownership of the object is distinct from the ownership of the copyright (although museums have been working end runs around that for years). In general, copyright belongs to the creator, with the exception of what I think are called works for hire, eg where created in the course of employment. In any case, copyright runs for life plus 70 years in most countries (I think it's life plus 50 in Australia still), and requires no formal registration or renewal, or copyright notice. (I'm skating a bit over 20th century US copyright here, which lasted for shorter terms and did require renewal, but was possibly "grandfathered" into the new copyright regime.)
I think, if you come across a 35mm Kodachrome slide, the odds are it's still in copyright. E6 slides, even more likely. The only way to know for certain would be to identify the creator and find out when she died.
All that said, a good way of thinking of dealing with copyright is a risk management game. If you're not making money from it, and are prepared to take it down if the true owner turns up and claims it (relatively unlikely for a found slide), then the downsides are pretty small. One way round is to post them with a question asking who made them, so you're genuinely trying to help the original creator or their descendants re-unite with the work.
Sorry, more than you're likely to want to know about copyright at 23:30 on the day England was happy to lose some sort of strange game of footpingpong...