Ok need someone with better legal knowledge than I have:
If police visited the Spedding home after BS had left to chat with Mrs. S, and possibly left some recording devices around, don't they need a warrant for that so any evidence would be admissible? If so, then they must have probable cause? So a lot they are not saying and keeping close to their chests. I believe they're trying to rattle their POI and his family members.
I also thought it was odd the way Mrs S worded her reply about the original search: "I am confident because they won't find anything related to William" or words to that effect. Surely most people would say "because he's innocent!" not imply that he had covered his tracks too well.
Yes, they do need a warrant for any listening devices - and they don't dish them out willy nilly.
http://www.austlii.edu.au/au/legis/nsw/consol_act/sda2007210/s17.html