Synology NAS: spurious message "Display all <n> possibilities? (y or n)" in PuTTY Terminal

I'm getting a "Display all possibilities? (y or n)" dialogue in a PuTTY Terminal connected to a Synology NAS. The Synology Support people are abdicating responsibility for this, so I'm wondering if someone can help to identify the source? I'm connecting with a PuTTY Terminal, using the Default Config: just login with user & password & wait...
...after a long wait (an hour, say) the "Display all possibilities" message appears. ps -ef suggested the Session is running a Bourne Shell ("sh") but $SHELL --version yields:
GNU bash, version 4.4.23(1)-release (x86_64-pc-linux-gnu) The reported OS is: Linux 4.4.302+ x86_64 There were no user-written scripts/processes running on the NAS. Pure Synology. During the wait, no input of any kind was made at the Terminal.
In that time, the NAS Filesystem was not accessed.
Neither was the Synology Console used (although it was open, displaying the Control Panel App). Is there some way my own Session can be issuing a Command without my knowledge?
(there was absolutely no spurious input - Tabs, or otherwise - after the login) Could some other Process be displaying the message on my Terminal? I'd be grateful for any ideas how I can get to the bottom of this and/or put a stop to it. Thanks in advance. **Screenshot 1, after login:** **Screenshot 2, after long wait:** **Screenshot 3, after entering "y":**