Just curious, did you do a 1/2 hr? I assume so based on what you wrote but wasn't 100% clear. As far as stealth, I don't go here anymore, but when I did, I found the best place to park was on the right side of the building (if you are facing it from 252) in front of the "small field". If you park there, you can exit without a lot of eyeballs and walk along the backside hugging the building to the stairwell. Yea, people from the gas station might see you or someone else, but they (mostly) have no clue exactly where you are going because there are a lot of businesses in that building. Unless you are a customer there you likely don't know that the stairwell leads to Yao.
I think you are pretty safe from eyes if you do it that way (and wear a mask and sunglasses or a hat, always helps). Glad you had a decent time here for a change!