This is a screencap from when Sherlock and John arrive at 221B in ASIP.
This is a screencap from when they leave the flat. We can see that the same blonde woman walks by Sherlock and the flat, but she's with a different friend now.
Clearly those three women work for Mycroft and are keeping an eye on Sherlock for him. (Maybe this isn't what they imagined they would be doing when they started working for the British Secret Service. . .)














