Trigger Warning
My stepfather was like this. Constantly checking, correcting, sneaking up on us to catch us out. Creeping up the stairs and opening bedroom and bathroom doors to make sure we were quiet/washing/brushing teeth/asleep, whatever fit the circumstances.
Very early bedtimes (compared to other kids on the street, no talking, whispering, quiet reading allowed. Had to be asleep before he went to bed.
Grassing us up: she did this, she didn't do that, they're constantly talking, she got out of bed etc. etc.. Mother always taking his word as gospel although it was mostly lies.
He went to bed earlier than my mother because he was up at 5 and worked ever so hard don't you know.
Then he began to come ever so stealthily into our room. If myself or my sister were awake he would tell us off in angry whispers then go down and tell her we were talking (we weren't, we never did when we knew he had come to bed at 9pm ) and he had had to come in to tell us off.
We learnt to pretend to be asleep. That's when he started to molest me by slipping a hand under my covers. If I stirred he would say 'there, there, you've just had a nightmare', then go down and tell Mother he had to comfort me because I'd had a nightmare. It progressed from there. If ever she questioned why he had walked from their room to ours there was always a plausible reason.
The control/checking started at 6/7, then he progressed to SA when I was 8.
To the outside world he was a loving SF. Took us out for picnics, to a large park a few miles away, walks in the woods, visits to the pony field a few miles away. The pony fields were when he pounced. He knew I wouldn't 'tell' because I was obsessed and that would stop the visits. He groomed, he said things. I knew it felt wrong but didn't know why. By the time I was 10 he could molest me in a room with other people and no one would be any the wiser. The same at home. Pony rides (piggy backs) in the garden, tickling games, cuddles on his lap, anything could be twisted. To everyone else it probably looked like we had a great relationship. That's because I was a child and was groomed, because he did fun things as well as the odd feels wrong things. Because he's driven such a wedge into the mother/child relationship that he seemed to be the only one on my side most of the time. Oh yeah, he would also tell me that my mother was fed up of me, sick of me and my behaviour, that she wanted to send me away but he'd talked her out of it. No one ever knew about those conversations because how could I as a child bring them up and question adult authority.
All planned, all premiditated, all very clever, and no one, least of all my mother, had a fucking clue.
Now maybe you've just got yourself a common arsehole OP, maybe he's not a predator eyeing up one or both of your children. Maybe.