It flew off, defying attempts to control it, and headed for a watery demise.

"It mysteriously crashed into the bay ... and sank," said Sinfield, a television news assignment editor who in the past few years has used his drone to chronicle the valley's tech boom, posting his personal flyover videos on YouTube.

With amateur operators increasingly buzzing corporate headquarters, sports venues and the private homes of celebrities, emerging counter-drone technologies could soon spawn an intriguing cat-and-mouse game in the region's increasingly crowded skies.

"You can't just put a fence around a property anymore," said Patrick Egan who teaches drone video production at the Academy of Art University in San Francisco. "You have to wrap it up completely."

Egan said many Bay Area tech companies go out of their way to protect their privacy, "so these flyovers are probably really getting their goat."

That's where tools like "geofencing" come in. Using GPS or radio signals, the technology essentially creates a virtual barrier against intrusions. Software that prevents a drone from breaching a geofence can be embedded in the craft's navigation system, which is what drone manufacturer DJI does with its "geospatial environment online" feature.

There's currently no law prohibiting a company from setting up a geofence around its property. DJI's software constantly updates where users are prohibited from flying over and operators are warned when they approach no-fly zones. A DJI drone will not fly into or take off in areas that raise safety or security concerns.

Robi Sen, founder and chief technology officer of a Maryland-based communications and security company, said some tech companies also are using "machine learning to help differentiate objects in the air so they can say, 'Hey, this is a drone.'" Other companies are using acoustic technology to identify unmanned aerial vehicles, he said.

"These tools are being used by prisons and at events like the Boston Marathon," he said.

Much of the anti-drone technology is being developed in secrecy for the government, so it's hard to know exactly what tools companies might be using to stop drones.

"If it's a jamming technology, I'd worry about violating FCC rules," said Brendan Schulman, DJI's vice president of policy and legal affairs. "And taking over control of a drone sounds like hijacking to me."

"If you're operating a drone for recreational purposes, you don't need authorization from the FAA to fly," said FAA spokesman Ian Gregor. "But you must fly safely and you can't endanger people or property on the ground."