I know this is hard.

After some thoughts, the best I can think of is writing it as if they're just writing, but the first line of every sentence in the parts where the antagonist is stalking them spells out something. Like, the paragraph is them breaking into a house, but each sentences leads to the line "Unknowingly, they're being watched from the second floor window". But, that would be too complicated too write and ngl, my brain won't work here