Since the latest update (15.1.2) the hidden characters feature is working very erratically. Characters appear/diasppear depending on zoom level (see screenshots at 100 and 125%). Some are not shown at all. In the example here, there are multiple spaces and a tab missing. This problem persists across all documents, also after restarts.
Is this happening in one file, or in more than one file? If one file, there could be some corruption in the file. This can often be fixed by saving the file as an IDML (File > Save As > InDesign CS4 or Later [IDML], then reopening the file.
If more than one file, try restoring your InDesign preferences: