Fair warning, I'm the jerk who wrote those dang scenes with Pandora, and even I got a bit teary eyed looking back at them.
Wait, should the fact that I wrote it make me more or less vulnerable to feels? WHATEVER! Fairly warned be ye, says I.
Less vague than I realized
I was considering leaving it more up in the air when exactly Pandora gave Sam his magic mark, but then I realized just how telling the placement of Sam's magic mark was, and that Grace would realize that.
Granted, Grace has less information than we do, and she's making a bit of a conclusion jump in panel six given her more limited knowledge of where Pandora has placed magic marks on other people. That doesn't make her wrong, but still.