Bay seniors with a fat tax refund and an urge to get away mentally but not geographically, should check out this secret 3- bedroom waterfront home in Tiburon. Keil Cove is at the eastern tip of this Marin peninsula, dozens of mostly empty green acres facing Angel Island and the Bay beyond it. You may feel you are in another world but this is Marin so there is a hot tub on the deck and a charge of $375 a night. On the other hand, for $000, the Tiburon Uplands Nature Preserve nearby provides even higher, wider, more Imaxish views of the Bay Area. But perhaps not for long. This precious lookout is threatened by the same genre of greedy developers who once concocted the idea of filling in Richardson Bay.