There are a few possible explanations if you've encountered something that makes you believe this is happening:
* DIY razor disposal: Some people may try to dispose of used razor blades by slipping them into small gaps in the mirror or wall. This is dangerous and could cause injury to anyone who later tries to repair or remove the mirror.
* Old construction: Older buildings sometimes had built-in razor disposal slots in the back of medicine cabinets. These slots would lead to a cavity in the wall where the old blades would collect. If you are in a bathroom with an old medicine cabinet and have seen a gap in the back of the mirror, it may be an old disposal slot.
* Vandalism/Malicious Intent (Unlikely): It's important to note that there have been false social media posts about razor blades being placed in public places with malicious intent. However, these claims have been debunked and there is no evidence to support them.