What Is This?

A bag filled with clear-liquid push-pinned to a finished header?

I've seen a lot of head-scratching stuff on the real estate circuit. I've uncovered weapons and drugs, lots of pornography. A couple of times I've stumbled onto cock fights or cock fight facilities. Family members sleeping in hallways and closets. Bomb shelters and full scale basements. Houses in which every available inch of wall space is consumed by televisions. Vegetation growing into and through the structure. Old cars mothballed for decades. Hidden passages. Animal remains.

Does anybody know what this is for? Incidentally, the house was empy.