Which parameter does InDesign (CC 2015 Mac) check to conclude: the resource to this link has been changed?

Indesign CC2015 / Mac

For about 3-4 weeks, individual (and changing without recognizable pattern) links on a webdav-server (that we have already longer and unchanged in use) are marked as "changed" (yellow warning in the link range) - without that we changed it and without any visible items have changed on Finder level.

We have checked all the parameters, but can not find anything.

Now we wonder if maybe InDesign checks other than the parameters accessible to us?

Does somebody has any idea?

Thanks + greeting

frank