Confirmed that the second had been muggled [V] then I spoke to security and they told me how long and how obvious the 'stealthy' search had been - they didn't take it! A pity but we have only lost 2 container so far so I will replace in due course.
I see one listing where the cache is a popular hangout and has observant security guards and closed-circuit cameras. This is a recipe for the container being removed any time immediately after a cacher finds it. Doesn't matter who the cacher is, although the timing of the hunt is a factor. I can't imagine uncomfortable cachers being questioned by a security guard, then stole a Geocache at that moment, nor returned later to risk facing another suspicious security guard. But I can think of a couple of ways the container could go missing in this situation. As mentioned, it may not be gone.
Give it a few weeks, then hide a new container. You should mention that extra stealth is required. I would avoid hunting a cache that's under special scrutiny, where the CO suspects me if anything happens to it.