The way I see it, if you find evidence of a bandicoot digging around in a bioretention system it's probably a good sign!
Why is that?
Bandicoots dig for food. If it's digging in your bioretention systems it's either (a) desperate; or (b) finding food there, and that's a good sign. Bandicoots eat insects, grubs, tubers and roots (amonst other things). All of these point towards a diverse, healthy and thriving filter media environment.