As way of an intro to set the scene. (maybe some of this info may be pertinent).
The house is a 50yo, solid brick house built on concrete beam foundations with a concrete floor poured in after the walls wereup (strange method, but that is the way the built it). I am the original owner who witnessed the build and have lived here since the houses completion.

The situation is that in the (only) bathroom, there is a cast iron bathbuild into a corner of an external 2 walls of house with the remaining 2 sides sitting on traditional dwarf(?) brick walls These dwarf walls are in turn tiled with 6 tiles. As part of the tiling there are 2 of, 6 x 3 air vents, set 12 off the floor. Each tile consists of 5 vertical gapsin the ceramic.

Today, one of the vents had a reasonable pile of dirt below it on the bathroom floor, as if the dirt had been pushed out the gaps in the vent from the space between the bath and the dwarf wall. The othervent has a similar pile, but much smaller. In all, about 1/3 a cup of "stuff" was on the floor.
On close inspection, the "stuff" consisted of fine sand and small lumps of building debris, like old mortar, brick chips etc.
Ihave never seen such before.
Upon closer inspection, prodding through the vent and the use of a vacuum cleaner to suck up the debris, it appears as if the gap between the bath and the dwarf wall is completely filled with this debristo the point where it has filled up to the back of the vent and overflowed onto the floor.
Another member of the household apparently saw a small amount of dust in this location a day or so ago.
To my understanding of how these baths are installed, the bath "hangs" on the dwarf walls on each side, with free space all around.

As far as I know,there is no other venting/access/holes to the area immediately surrounding the bath, and we have not seen any small creatures around.

Has anyone come across such a situation before and can suggest a cause, or can laya guess/suggestion as to what might be a cause of my mysterious sandy dust/building debris?