Mystery filesystem remount

[fluffysheap]

I use Steam, which I installed from the steam-overlay (games-util/steam-launcher). I'm posting this in security rather than games because the question is not about playing or installing games but rather some security implications I don't like. Whenever I run Steam, I get upsetting messages in my dmesg log, which indicates that somehow Steam is able to remount filesystems:
[270732.348534] ext4 filesystem being remounted at /newroot/mnt/extra supports timestamps until 2038 (0x7fffffff)

I understand why the filesystem has limited timestamps (and I don't really care, I'm sure I'll retire that disk before 2038), what I don't understand is how Steam is able to remount the filesystem. I don't have any of:

systemd
udisks
any setuid programs in the steam-launcher package
'user' option in fstab
passwordless sudo

At first I assumed this was happening through udisks, but I got rid of udisks and it is still happening.

Can anyone give me a clue on how to track down how this is happening? Or is there some other way the message can be triggered?

[alamahant]

Which partition is being remounted do you think?

[fluffysheap]

It's /mnt/extra, a partition I use for storing bulk data (ISOs and the like). It isn't a partition that's relevant to the system (or to Steam) but it is the only one I have that is formatted with small inodes, so the only one I'd notice a remount of in this way.

/etc/sudoers.d is empty. Also, steam runs as the logged in user, not under its own user account. My login user has a lot of permissions, but it doesn't have THAT permission (I have to use sudo to mount anything, and I'm not a member of 'disk' group).

[Hu]

Perhaps steam is playing games with user namespaces, chroot, and bind mounts.