I have two that I found locally and put in a tank to investigate/observe... I quarantined them for a while because I read they are very susceptible to diseases and parasites. For a long time I thought they were dead, but they kept moving around ever so slightly and there was nothing big enough to move them in the tank.
They have grown some and apparently they move around but I have never caught them in the act. I would venture to believe they prefer mud and silt over sand or small gravel...
I'm proud that I've had them as long as I have without them dying or killing everything else in my tank.
But to be honest, until I saw this thread I haven't thought about them in a month or so... They are great pets for the absentminded and unaware, I suppose! (smileythingee here)