This is the second time I have had to uncover the water meter and valve in the access box where the water main comes into the house. I am unable to diagnose nor explain why dirt, gravel and sand periodically have filled the access box. I would say some critter is doing it, but I find no evidence of burrowing or nesting. Moreover, I cannot see how a small critter is able to move the relatively heavy metal lid that covers the box.
I got the garden trowel and dug a bucket of dirt and stuff out of the box, just enough to uncover the meter and valve. I replaced the cover and put a large rock on top of it.
I took a before and after picture of the water meter and valve access box which are embedded into the two-frame slideshow above. Click to advance.
Maybe javelin rooting around. I would guess they can smell water and would expect the scent is there even if only a minimal amount may escape the meter by evaporation at the pipe joints. Of course, it could be your utility meter man screwing with you but my guess would be javelina. Do you have a game camera or know someone from whom you can borrow one. Set it up near the meter box and see what it captures if anything.