i have had this, though variations throughout my lifetime. sometimes, it has been the feeling of being watched externally, so more the anxious/paranoid type. other times, it is what you describe without anxiety or paranoia and like i'm being watched from the inside, just under the surface type thing but don't really always know it, though i do type of thing.

when i was younger, before i was able to put pieces together, i thought things were coming from outside of me. once i realized it was from inside, it helped a lot, and some things settled down after that. i wasn't crazy like i thought, at least not in that way lol but things have varied for me in that regard.

i've had all kinds of 'being watched' experiences (for real and not real), so it has been a process when each thing has come up as to how it manifests and figuring out what, why, etc. and then having it kind of lessen again.