Generally when the tray icons there, it means Hard Disk Sentinel could not close itself correctly, like if somehow terminated (like if something may forced it to close).
To investigate, would be required to "connect" this event to something which may happen with the system.
If running 24/7, then power management (entering/leaving standby, hibernation or similar) should never occur, so there may be something else.
Please try to observe when this may happen actually, for example after connection/removal of a drive (removable device) or something similar, just to verify if something is related.
Please use Report menu -> Send test report to developer option. Also if you see LOG.TXT file in the folder of Hard Disk Sentinel, please send in e-mail to info (at) hdsentinel (dot) com
These may help to check the actual situation, examine the configuration and may give ideas, thoughts - at least to try for further investigation.