Re: Theres a lesson here..
I did the same thing when I left my last employer. In addition I handed back my company phone and got them to chop up the SIM. I also got the IT admin people to disable my account on the day I left so that I could 'test' that I could no longer gain access to the company's IT systems before I walked out the door (with a spring in my step). I got them to document all of this.
I went through this rigmarole because I had witnessed a completely innocent ex-employee getting accused of fcuking with the IT systems after he had left the company to cover up a horrendous cock-up that occurred in the department he had previously worked in. Just before this was go to court the company backed down and admitted that he was completely innocent and blameless.