VDI VM shutting down after logout

Hi,

On one of the View infrastructure i manage I have a strange issue since one week. In the morning i find between 1 and 5 VM (out of 15) shut down in VSphere client. According to the log the shutdown is initiated by "user" so not by the view server nor the admin using vsphere client. Also according to the log the shut down occur around the time by which the customer stop using his VDI fo the day, when he leaves the office. They say they just close the view client soft, and they don't have the right to shut down the VM anyway.

Any clue what may cause that ?

Thanks