nautillus mounts nfs shares when searching

Thank you for your bug report, that's an interaction with autofs and not really a bug (the search needs to read the directories to find content), you could still try to report it upstream though, https://gitlab.gnome.org/GNOME/nautilus/-/issues

I haven't autofs installed:

$ aptitude search autofs
p autofs - automounter basato sul kernel per Linux
p autofs:i386 - automounter basato sul kernel per Linux
p autofs-hesiod - supporto per mappe Hesiod per autofs
p autofs-hesiod:i386 - supporto per mappe Hesiod per autofs
p autofs-ldap - LDAP map support for autofs
p autofs-ldap:i386 - LDAP map support for autofs
p autofs5 - pacchetto fittizio di transizione per "autofs"
v autofs5:i386 -
p autofs5-hesiod - pacchetto fittizio di transizione per "autofs-hesiod"
v autofs5-hesiod:i386 -
p autofs5-ldap - transitional dummy package for 'autofs-ldap'
v autofs5-ldap:i386 -
p fusiondirectory-plugin-autofs - autofs plugin for FusionDirectory
p fusiondirectory-plugin-autofs-schema - LDAP schema for FusionDirectory autofs plugin

Could you add the output of 'mount' once those are mounted? nautilus doesn't do any automatic mounting so it has to come from some lower level feature enabled on your installation

Here are the relevant lines of the mount command:

nas:/data/Documents/snapshot on /media/nas.nfs/Documents/snapshot type nfs4 (rw,nosuid,nodev,noexec,relatime,vers=4.2,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.1.3,local_lock=none,addr=192.168.1.11)
nas:/data/Documents/snapshot/c_2016_10_31__00_00_21 on /media/nas.nfs/Documents/snapshot/c_2016_10_31__00_00_21 type nfs4 (rw,nosuid,nodev,noexec,relatime,vers=4.2,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.1.3,local_lock=none,addr=192.168.1.11)

trying to umount the inner one:

paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
umount.nfs: /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11: device is busy
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
umount.nfs: /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11: device is busy
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
paolo@paolo:~$ sudo umount /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11
umount.nfs: /media/nas.nfs/Documents/snapshot/c_2016_11_30__00_00_11: device is busy

sometimes it says busy, sometimes it doesn't say anything, but no umount is done

I get everything umounted with

$ sudo umount /media/nas.nfs/Documents/snapshot/

I must be root: from nautilus the eject button says "operation not permitted", but why? nautilus mounted it, and it cannot umount it!

Could you try if

$ lsof | grep snapshot

gives you some hints on the processes that block the unmounting?

sudo lsof | grep snapshot

says:

lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
      Output information may be incomplete.

and only reports lines which relate to chromium-browser or darktable

weird, does it mount if you just 'cd' to the directory on a command line or use 'ls -R' in the snapshot directory or is it really specific to nautilus?

actually, cd'ing into the directories mount them, so it's not nautilus

But, then, who could be mounting them?

Unsure, that's a feature autofs provides
https://help.ubuntu.com/community/Autofs
but you said you don't have that installed...