hi Marg - have you watched the specific discussion?
Around 12:13 - it really doesn't seem like a matter of having bugged him prior to the reporting of her disappearance. If they had, surely they would have 'pounced' much sooner than an entire week after she was potentially killed.
If you watched the press, it is very clear that they moved to look into him as a suspect after his version of events did not corroborate what they were able to track by "electronic means", which includes:
* Logs of phonecalls between JS and KM
* GPS data of movement to various locations and shops
* CCTV at shops/petrol stations purchasing certain items, and shop records of what was purchased
I don't think it is helpful to suggest that the police were across this a full day before she was reported missing. I would urge you to watch the specific discussion which does not suggest at all that KM and JS discussed the sandbags - it is clear that they are aware only that there were discussions.