You're not alone! DS is 20 months today, and he's been doing the sitting up thing for months now. He never makes a sound when he gets up, just sits there. The days I go to work (Tues-Thurs), I get up to work out at 5 and we don't go in to get him until about 6:30. Since I'm up, I bring the video monitor with me into the workout room. There are days I see him sitting there, just chewing on his blanket, sometimes for up to an hour or more. Sometimes he'll lay down after sitting up for an hour, sleep for about 30-40 minutes, than get up again (for the day).
Same with naps - sometimes, he'll sit up after an hour and a half of napping, and he'll sit there as long as I let him (usually during the day, I don't let him sit more than 30 minutes, at which point I realize he's not laying back down).
I'll watch him keep looking towards the door, so I know he's waiting for me or DH to come in and get him. When this all started, I told myself I'd watch him closely, and the times that he sat up when it really was time to get up, I'd go right in... and if it wasn't time to get up, I'd leave him alone. I thought that would "train" him to realize that if I didn't come in soon after he sat up that I wasn't coming in yet, and he'd lay back down. That didn't work for two reasons: 1) Like I said before, sometimes he'll sit there for so long that I have no choice but to go in and get him because it's time to get up. In those cases, I think he thinks he waited long enough and I came in. 2) I've figured out that even if he's just had a 2 or 3 hour nap and then sits up, if I go in within the first 5 minutes or so of him sitting up, he gets REALLY upset, like he hasn't really had a chance to wake up yet. Yesterday, he slept 2 hr 45 minutes, and after I saw him sit up, I went in a couple minutes later. When I turned the light on, he started crying and didn't stop for 20 minutes. So, I have to give him that time to get "uncranky" and awake.
Anyway, I don't know what to tell you except just take it day by day and trust your instincts. Most the time, my gut tells me to leave him alone since he's not upset, but I agree with you that sometimes I just feel so bad for him, sitting there watching the door and waiting for me.
When I hear people talking about their kids that sleep so well but don't have a video monitor, I wonder how they can be sure they're really sleeping the whole time... They say, "It doesn't matter if you know or not, just don't go in there until it's time for them to get up." That's a nice way to think, but if my son is quiet in his crib for 12 hours straight but didn't sleep the last 2 hours, that makes a difference to me! I'm anal about his sleep, though.
Good luck and sorry to ramble in your post!