I had sub assemblies disappear in front of my eyes as soon as I open them with the parts missing from the tree. And I'm the only one working on them. Our Solidworks service provider says at times assemblies can get corrupt while saving over the network. The PDM is suppose to prevent this.
Julie Advocate,
Are you running 2017 pre sp3? If so, have a look at With SW2017 SP2 we are seeing frequent corrupt files in assemblies.