I believe I know what you mean. There is a glitch where sometimes people will log out of PSN, but still show up in your friend list as being online. It has happened to me in the past, and has caused some problems. You can log out if you suspect this, then back in, and it should show you who is on at the time.
Thanks Asuka424 for the Signature
Are you noticing this on your ps3? I've had an issue on my ps4 and vita, when my friends go offline, or switch from home to game, or game to game, it'll continue showing as if they were doing the previous thing. Like, if they were on home, then switched to a game, it will still show them as if they are on home, when they're not. It's weird.