We sometimes think of our memory as an hard drive we can read files from with an index, that's really not true, our memory resembles Content-Addressible Memory, where the data itself (of at least something similar to) is the key to unlock the data. You recall a piece of memory only when you are epxeriencing something similar. This also explain the "unlocked memories" of things you did in the past you completely forget until you find an object related to it.
The mind is fucking weird.
Yeah that's called an intrusion, hackers do that and it's illegal. If you accidentaly leave you house door unlocked is it your fault if someone trashes your house?
Report them, no damage was done and it's a relatively minor thing so I wouldn't expect grave consequences, but maybe this person will be more more responsible in the future.