Selected Answer
Essentially, if you are able to open a file while not connected to the network, the file must be saved on your computer. If you are able to open the file from another location, the implication is that a copy of that file exists somewhere else, a location accessible from the network. Finally, if the file you open from another location is identical to the one you last saved on your computer (probably laptop in this case), there must be a program which keeps the two copies of your file in sync.
Therefore you should look for an explanation for your experience in the syncing process. You might, for example, have shut down your laptop before the syncing was complete, or the network connection wasn't stable at the time. Something prevented completion of the syncing process.
When you tried to access the file again the system detected this deficiency and gave you the grey X. However, the system is also built to normalise such situations. Probably, after you reconnected to the network the syncing could be completed and the file was released for normal use as a result.
