More weirdness! After further testing, it turns out that this behavior only occurred in files on some of my drives and not others. I determined that the common link (drives that did versus drives that didn't prompt) was Amazon Cloud backup running in the background on the drives that did. I'm not sure what Amazon Cloud is doing to the files, maybe setting an archive flag or something that's telling Leo that the file changed.
Anyway, problem solved but thought I would share the details in case anyone else might run into the same problem. Thanks for your suggestions, though.
Rob.........