If someone has access into your Mac, they can do whatever the hell they wish to do. They can also install whatever kind of malware they want if that's their desire (provided they have enough computer knowledge to do so).
The best way to prevent this is to not be stupid and 1) put in a fairly secure password 2) don't spoil the password in the password hint thing 3) recommended, enable FileVault so that nobody can get to your data unless the password is known.
And obviously, don't be stupid and don't leave your computer in a public place alone for someone to infect I mean steal.
If someone had access to your mac, is it possible for them to install any type of spyware without you knowing?
So, without the password, there is no way someone could have installed anything?Download and run Malwarebytes for Mac that will detect & clean any malware. I wouldn't dwell on it as an individual would also need your account password to install an application on OS X. Personally I also set my display to lock after a short period of time, just in the off chance I do have to leave the notebook unattended.
Q-6
So, without the password, there is no way someone could have installed anything?
Also, wouldn't there be traces of said application if such existed?