Default theme: images disappearing for no reason

[LiquidFractal]

Here's something that I've had happen a couple of times in the past, and logging in to my website this morning I find it's happened again:

I go to the admin signin page, and find that the Invision logo is missing from above the Display Name or Email Address and Password fields.  I can still sign in without problems.

There are other images missing from the default theme as well: in particular, the default Guest placeholder image on the Who's Online page (yoursite.com/online).  This is the only other missing image AFAIK but I haven't looked closely at every page.

I use a custom theme so this doesn't directly affect me, but it annoys me because 1) I don't know how/why this happens, and 2) I don't know what else might be missing.  If I ever want the default theme available to clients/members, this needs to be addressed.

This has happened across two VPS (my previous one and my recent new one), so I don't think it's a specific server issue.  The only thing I was doing on my site directly before this was updating apps and plugins.

Has anyone else had this happen?