Yes, the toll signs are plainly displayed, and at least two on any approach. You can fire up Google Earth or Google Maps, drag the "little man" figure to any of the blue lines on the map, and enter Street View (which are photos taken from roving automobiles with special cameras on their roof) and travel US-101 or Doyle virtually and see the signs for yourself.
Your answer is that you missed the signs. That is either because you have not developed the habit of reading signs, or the approaches to the Golden Gate are fairly high on "pilot workload", and you may have had other safety tasks which took priority for your attention.
Signs in the US have several color schemes:
- Red, Crossbucks, and Circles - urgent warning signs
- Orange - construction zones
- Yellow - important/compulsory navigational signs (e.g. Merge or Curve Ahead
- Green - navigational signs, which you can disregard if you know the route
- Blue and brown - roadside services, attractions and points of interest, which you disregard if your trip is planned.
The signs describing the route to the Golden Gate Bridge are green or blue, but all the toll-related signs are yellow, often an inset on a green or blue sign (which itself is notably weird to the experienced eye). Part of the rhythm and flow of driving is learning what to ignore; I could see where someone unpracticed in US driving could overload on the many blue, green and brown signs in the area.
What's more, most people use nav's ... And they neglect to tick the "Avoid Tolls" option. Some navs present a no-toll option as one of the three choices. However in this case, the shortest toll-free option is via I-580 and CA-237 in Milpitas/San Jose, not really viable.
We often get "Was this toll entrance properly signed?" I can tell from direct experience that I got out of a carpool lane ticket when signage was wrong by design. So it could happen, and I approve of arguing the point when it is so. It just isn't so here. You must pass and disregard at least 2 signs on every approach to this entrance.
One thing toll districts are starting to do is use purple signage. But even then, people swear they didn't see it.
"License plate scan without penalty" is the normal fare collection on the Golden Gate Bridge, and other systems are going to it as well because it's cheaper and faster than staffing toll takers. Golden Gate Bridge adopted this early, because there is simply no sane place to put the toll booths; the old toll booth setup was a dangerous nightmare.
Now some of these "license plate scan without penalty" systems have a discount for operating with a toll tag, and in some cases (Chesapeake crossing) the toll tags empower further multiple-trip discounts which can be substantial. Some people call this discount a "penalty against those who do not qualify for the discount". I think that's silly.
I also don't take much stock in the argument that "It's a huge, iconic suspension bridge that crosses a major frontier, how could anyone possibly imagine that it would be a toll crossing?"
Get used to the new world: Toll collection isn't like it was. If you have any reason to expect a toll, and you certainly did here, it is your responsibility to either a) figure out on advance how those tolls work and how to pay them, or b) be diligent about searching for "toll road entrance" warning signs. This is on you: if you don't do it, expect the nasty letter in the mail.
The toll collectors are really trying to help, e.g. by implementing "any car welcome, pay by plate" methods instead of $25 or $50 penalty whammos. The Golden Gate Bridge has signs advising you to pay by Website/plate in within 24hr of crossing.
A few people also scoff the law, on the belief they are untouchable due to being in another state or country. That won't work, and will get you a misdemeanor conviction in CA. But I have to say the US is on the back foot morally, since our diplomats refuse to pay tolls in foreign capital cities.