once at a large hotel, ground floor room facing the main walkway. Mid-afternoon with the drapes wide open to let the light in.  

We didn't realize the windows weren't mirrored until we left after the session was over. Evidently no one complained because we weren't interrupted.

in the past

My kinky SO always did when we traveled, it was always very erotic to think you may be watched. Didn't you ever cum across other couples doing the same thing? I have some great home movies of such events.  I don't know where all that free wonderful sex went?

but I grabbed the curtain rods and pulled them shut.

I was seeing a "Traveling Provider" and she had a room that overlooked the city. It was dark outside, her curtains were open with the lights on.

It wasn't planned, but somehow she ended up bent over the chair in doggystyle (with me of course behind her) looking straight out the window. Great memory! And as far as I know, anyone could have seen us

This past January I was with a great provider in her NY hotel room and she left the drapes open.  There were windows just across the alley from which anyone there could have seen us "in flagrante dilecto," but I never saw anybody.
A lot of this was going on at a new hotel in Manhattan's meat packing district called The Standard.  It was built with floor-to-ceiling glass for the outer walls of each room.  It also was built right over the High Line, an old elevated rail system that's been turned into a pedestrian walkway and park. So people at the Standard started fucking up against the glass windows so strollers could see them.  It became such a well-known spectator event that The Standard put a card in each room warning guests about losing their privacy.  When my SO and stayed there a couple of years ago we had sex, but not up against the window.

Maybe if I had someone with me  they would have been more receptive...

The windows had vertical blinds half closed at an angle... we were busy breaking in the furniture in the living area and then dress and left for dinner. When we came back we noticed that the view out the window by the elevator was straight into our room and the couch, because of the angle of the blinds. Everyone checking in had a view! LOL

... on any of the windows in her apt. It was a 5th floor walk up in a neighborhood of similar buildings. The chance of being seen from the street was there but not nearly as much as being seen from an apt across the street. I like my appointments at mid day so it was not nearly as bad as at night when the apt is illuminated by interior light. I wondered "What the fuck is she thinking. Or is she even thinking at all". But hey, it's a weird/fun game.

on TER, of people doing it in front of an open window or right out on the the hotel balcony and stuff like that. (Of course reviews are fantasy)

We were having sex with the drapes open on the 20th floor.  She then mentioned there was a group of people in a hallway window across the street watching us.  I didn't really care either way, but she seemed to like it.  She was also an ex-stripper.  Maybe she missed the attention.