I wasn't really concerned before I made this thread, the other users here warned me about the dangers of leaving a computer unattended, especially to a technician. I wasn't too concerned about my Metamask wallet, since it's encrypted and requires my password, the Bitcoin one though was at a higher risk.
Thinking back to it, i shouldn't have handed my laptop without removing the HDD, which was merely a two minute job.
Yeah, my personal rule set is never include your hard drive with the laptop when you're offloading it to someone else. You don't know what people's intentions are, though most will likely be innocent, and wouldn't even think about checking for previous data. Then, the more, and more they use it the less likely it becomes to recover anything substantial.
If you are purchasing old hard drives it is usually enough to wipe the hard drive via a format, and not worry too much about malicious code. Though, there is a small amount of risk involved with that too. If you're offloading your own computer, and the hard drive has to be included for whatever reason, then I would recommend writing 0's to it, with multiple checks. You can do this within Linux or you can use a program like DBAN if you wish.
Thanks for your feedback, it's definitely a concerning matter, to be honest, I didn't even think about checking the hard drive, when I bought this refurbished laptop, (HDD was used, so I would probably find something if I was willing to), but it didn't even cross my mind to snoop into someone's files like that.
I haven't made up my mind yet, on whether to sell it or not, but I appreciate your feedback regarding the HDD overwriting.