It is necessary to pay attention to the safety lever, even if one carries off-safe, when carrying a holstered weapon. Things can brush against the safety, changing its position. While walking forward, through vegetation, for example, that safety lever can be moved to the down-most position.
As the old saying goes, “don’t get caught with your Dingus down.” Another way to remember this is to orient safety so that it points toward the enemy. (Of course, the AR15/M4 selector is notable in that semi-auto “fire” is with the lever pointed downward, which caused me no end of cognitive dissonance.)
There is a universal thumb placement-and-movement, that minds the safety lever or most pistols. GRIZZ22 already mentioned this. This is important stuff, for defensive carry pistols, so I am talking about it, again.
Levers and buttons, changing positions, is not conjecture, on my part. One reason I really started disliking ambidextrous safety levers on 1911 pistols, for example, is that I found mine to have been brushed off-safe, more than once. I have found shotgun cross-bolt safety buttons having changed postions, countless times. I found the heel-clip magazine release, of an older-version SIG P220, to have snagged things, causing a partial mag drop, three of four times. And, in the days that I bought holsters, without paying attention to such things, some holsters will tend to press against a push-button mag release.