I think this no trespassing sign at the SAP Arena is brand new.
The back of the San Jose SAP Center is the popular and easy route for access to the Guadalupe River Trail from San Jose Diridon Station, the major transit hub located between downtown and midtown San Jose. This is the route often used by San Jose Bike Train, which runs the first and third Wednesday of every month.
I think (and I hope) this sign and any enforcement action is intended for the people I sometimes see parked on the sidewalk in front of the loading dock here. The Bus Rapid Transit Construction happening now directly in front of the SAP Center on West Santa Clara Street makes this bike-friendly detour especially important.