The mysterious RED X

Can anyone explain this one to me? I’ve noticed that sometimes my networked users(including myself) will see a red X over networked drives, yet they are still accessable. If you select a drive and wait a second or two, the X goes away. I’m puzzled by this. As I said… they have access to the drive and the server seems to be set up correctly, but still, the mysterious RED X
Perplexing! Any suggestions? Thanks!