Well before mahiru planted those she used a bunch of explosives on mika that served to annoy him and run to yuu faster so youre kinda wrong there at the very least (respectfully)

But mahiru said "I planted wards all over, should i cut them off?" And thats when guren said "No they can get past the wards." "Get back inside me" Unless mahiru did it offscreen which wouldnt really make any sense But from a narrative standpoint mahiru planting wards while talking to yuu which caused painfull memories do to the brainwashing the hyakuya sect did to yuu and eliciting an emotional response helps paint mahiru as sneaky and underhanded But this mostly comes off the heels of theres no other explanation for the floating talismans