I've been playing lately in a world populated by Sims my daughter and I have created or pulled from the Gallery, so Bob and Eliza aren't in the picture.
When I play a save with the premades, I see Eliza every time my Sim goes to the gym or a club. She's often in a Flirty mood. Bob emerges much less often. Once, when my Sim moved into a house near theirs, Bob and Eliza came over with the Welcome Wagon. Bob arrived in a terrible mood, ate food from my fridge, took a bath in my tub, and returned to the party wearing a towel. He farted loudly, said goodbye, and left, still wearing one of my Sim's towels.