Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In VN's novel Ada (1969) Demon Veen (Van's and Ada's father) learns about his children's affair by chance, because of Uncle Dan's Boschean death. Describing Demon's unexpected visit to his Manhattan flat, Van mentions a piece of nobler metal placed by Demon among the silver domes that Valerio (a waiter at ‘Monaco,’ a restaurant in the entresol of the tall building crowned by Van’s penthouse) brings to Van and Ada: