The photo shows factory original OEM sawdust created by Amish craftsmen. :M This was found in a cavity between the shower and bathroom wall in our TT. I've found plenty more in normally inaccessible places like the ceiling cavity and inside cabinetry as well as light fixtures, speakers and anything else that is screwed to the ceiling.
They simply don't vacuum up sawdust and debris. Where they hack a hole in luan, the cutout piece gets left in there. Have also found things like screwdriver bits, screws, cable clamps and bits of wire. Personally, I think it's shameful not to spend a few seconds vacuuming up as they go.
I doubt it's sawdust from ants, but not impossible. What you could do is go look for some OEM sawdust and compare it to what you have with a magnifying glass. You might find some under the tub (or shower) by removing the cover on the access hole. Ant sawdust should be much finer and consistent size.
![](http://i594.photobucket.com/albums/tt26/zippyfiat/008640x480-1.jpg)