As I said, it would have been incredibly difficult for the police to have placed a bug inside Lumumba's bar without their presence/activity or the bug itself being detected. It's far, far more likely in my opinion that the police bugged Lumumba's mobile phone - an astonishingly easy and undetectable endeavour for the police via the mobile network centre.
Furthermore, what would the police have been reasonably hoping to achieve by placing a bug within the bar? As others have pointed out, the noise within the bar would make it nigh-on impossible to understand any specific conversation. What's more, the quoted testimony strongly implies that it was Lumumba himself - via his mobile - that was the object of the bugging/tapping. Lastly, I would imagine that Bugatti only dealt with telephone intercepts - I suspect it would be a whole different arm of the police that specialised in placing listening devices within a premises (in the UK, for example, it's only Special Branch or the security service (MI5) that do this sort of work).
All the evidence points to it having been Lumumba's mobile phone that was tapped, and that the tap was placed one or more days before Knox's infamous "confession/accusation" on 5/6 November.