I think it is quite different between independents, micros and those who share an in call. I have my own in call, and keep low volume, with longer engagements. There is zero chance my friends would ever pass one another in the building, much less on my floor or at my place.
I believe that most of the independent ladies who share an in call or rent off of one lady adhere to a schedule as Erica and Miss Hunter mention, to ensure that discretion is maintained.
On the other hand, there are a lot of agencies that work out of condos. Many will market their ladies as being independent, or just as a group of girlfriends who are sharing a space. In those situations, the bottom line tends to be the most important consideration, and volume usually outweighs discretion. It isn't uncommon in those situations for 2 or more rooms to be utilized at the same time by different ladies and suitors. Essentially, if somebody wants an appointment, and there is a lady available, they will find a way to eke out the space to make it happen. In those situations, it is quite common to run into others in the hallway, or even in the apartment. Basically, the more ladies who have access to a unit at the same time, or with overlap, the more likely you are to encounter another friend.
For the most part, if you are communicating directly with the lady you are seeing, and she has her own phone number and website, your expectations for discretion will likely be met.