Mysterious co-worker

In the work area of the library where I am employed there are two private staff bathrooms. Both are located down an out of the way hallway that dead ends in a rarely used shower room. When one enters this hallway these bathrooms are to the left, and to the right is a sort of space tucked behind the hall entry where we used to unadvisedly keep the staff lockers but where we now just store... stuff. 

When I walk into this hallway heading off to, well, you know, I am surprised by a dark, looming figure standing creepily in the corner to my right. My heart jumps, skips, stutters. Is some mentally unstable co-worker waiting to leap at me as I'm going to the bathroom?

No.

One of the things stored in this space, hidden behind that small bit of entry wall, is some kind of folding contraption, maybe used for a portable library that my system is temporarily and inexplicably interested in. The contraption is wrapped up in a case. It all looks like a cello maybe, or even a bass as a professional musician would travel with, but with some extra stuff packed into it. If one looks at it directly it doesn't look much like a person of any kind, but if one is going to the bathroom, and thinking about other things, and sees it out of the corner of one's eye, it looks a lot like a mugger, or a bogey, or a sinister spectral figure of doom.

So my heart stops. "Yike!" I cry out inside. Or maybe more like "Aghh!" with a shiver.

Oh, it's just the bag, the bag that has been there for a year or two. The bag that I have seen 600 times.

One would think I would remember it. One would think I would grow accustomed after being startled 600 times.

No.

It's "Aghh!" with a shiver every time.

After 300 times I thought maybe it was something to do with me, how could I not learn and remember this illusion and fright no matter how many times I have experienced it? But after 600 times I no longer blame myself.

I have learned.

That bag is up to something. I blame the bag.