Not surprising ... stealing signs is an art form for some teams. Pitchers, in particular starters who have the game off, get especially good at stealing signs.
In the NCAA tourney, we had a different "Card" for each pitcher. If a pitching change was made, the coach brought out the new "card" and gave it to the catcher on the mound. Indicators for the signals to the catcher can vary by inning or count. You can have alternate cards for a pitcher and swap them between innings, making it even more difficult to steal the signs. Just make sure the coach and catcher are using the same card each inning/pitcher.
Next to impossible to steal, if done well. Only our catcher had a card and it was just used for pitches.
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We used to vary our indicator for base runners depending on who was on first. That works very well too and makes even a simple 3B coach signals very difficult to detect/steal.