Responding to reports about mountain lions sunbathing at the Fremont BART station, California Department of Fish and Game officials flew over the area with a helicopter and heat-sensing equipment but didn't find the offending felines.
When I first moved to Silicon Valley almost a decade ago... I spent a lot of time inside because of El Niño. But when it finally dried out, I dismantled the ark I'd been building, and took to hiking around beautiful Northern California.
At the time I was an East Coast transplant from Cambridge so I didn't take the warnings about mountain lions too seriously, and to this day (knock on wood) I've never seen one personally. But you do hear stories and the City of San Jose even publishes mountain lion and coyote safety tips.
There are noted attacks in Northern California, and though only 16 people have been injured by mountain lions since 1890, encounters have become more common as Silicon Valley's real estate stretches more into territory formerly claimed by big cats. The San Francisco Chronicle notes that animals typically try to avoid people but also provides tips in case you run into one!