The first thing that jumps out at me is that this could be a possible form of harassment. Even if the employee doesn't want to talk to anyone about it, if you know who it is, you can advise them that it has been observed that they are getting too close to other employees and its making people uncomfortable. They may not release they are doing it. Or, if they are, it puts them on notice.
Granted, there are times employees have to be close to one another, so that's going to happen. You may need to try to dig for more details. How is the employee getting too close, what are they doing while they are close, etc. You may have to tell the employee that you want to do something about it, but they need to provide more details and answer questions to make that happen.