It depends (but not new behavior, as far as I know)...
Animals can escape from traps, in which case they're triggered without anything in them. Predators can also eat animals caught in traps, leaving them empty. When birds escape or are taken by predators there are often feathers left behind, but I don't think that happens every time.
I think large animals can occasionally trigger traps too small for them as well, although animals usually treat such traps as blocking obstacles.
Edit: When I wrote the above I wasn't aware that the beta had been release (and didn't register the release number clearly on the subject line). Thus, treat the comment with big caution, based on the current stable version.