The wide shot at the very beginning of the opening scene in the club shows Piper and Leo talking with Paige and Dave in the bottom right corner, while Phoebe's at the bar at the top. But, it was shown before Piper and Leo found Paige and Dave...
Right before Phoebe turns into the superhero for the first time, she has her purse and a tape recorder with her. Those things disappear when she transforms and do not reappear when she turns back.
The first time the Aggressor comes after Remus, Paige orbs in about a second after being called, complaining about how close she was to orgasm. If she was having sex with Dave, there is no way she would have been able to make an excuse, get dressed, get to a place that Dave couldn't see her, and orb home within less than a second.