In the old days, in the 1980s and 1990s, scenarios never came with a map at an appropriate scale for use with miniatures. The GM's map would be complete, with secrets, at a much smaller scale, in the region of 0.25" on paper to 10' of character scale. For playing with miniatures it would be used like this:
The players would set up a generic marching order, using their figures, on the table.
The GM would progressively draw a map of what the characters could see as they explored. The earliest D&D rules had examples of the GM explaining what the characters could see and the players drawing the map, but most people found that this was too confusing, and had the GM draw the map for ease of play.
When an encounter started, the GM would either sketch the surroundings at the right scale for the figures, or set it up using some kind of tile system. I used Games Workshop "Dungeon floor plans," which were simply cardboard printed with squares over an appropriate background, which you could cut into appropriate shapes with scissors. There are examples of what they looked like here.
The players would put their figures onto the encounter map, the GM would deploy figures and/or counters to represent monsters, and the fight would proceed.
This worked well, at least in part because combat was much simpler. The grid was an aid to positioning, rather than there being hard rules about creature size and space occupied. There were no Attacks of Opportunity, and everything about combat was less formal.
I never ran across a GM drawing their own map in advance for a published scenario. There wouldn't have been much point to doing that, unless the GM wanted to re-arrange the scenario significantly. I have seen GMs at conventions, where they could have a large table, with a pre-drawn map at figure scale, which they would reveal piece by piece by removing sheets of paper that they had placed over it. However, this is surprisingly cumbersome if the map is at all complicated, and always seemed to me like too much work for the (minor) improvement it made to the game.