Resolved [Is it possible to place files on a computer without the owner knowing?]

A "true pro" won't get into your network or computer IF:

1. the router uses a strong password
2. wifi uses WPA2 & a strong passphrase
3. public folders are not shared
4. file & printer sharing is OFF
5. remote desktop is disabled
6. computer is virus/malware free

That leaves social engineering as his last resort. This includes phishing, physically monitoring keystrokes, covert casual conversation, reading your trash, etc.

It also assumes that the "true pro" does not have access to ISP data centers or govt monitoring facilities.