To answer your question, why do they keep you as a contact after they leave?
I'm going to try and not sound harsh but it's because the borderlines fear abandonment, both real and perceived. It's a way to manipulate you in the future and see if you still care about them. Usually they have several of us nons as backups (my ex would never go a day without already being in another relationship or at least someone’s bed after a breakup). They cannot be alone as this is their biggest fear. To not keep your things or refrain from contacting you in anyway would mean giving you closure and then they might have to face the fact that their perceptions that you might not actually care about them are true. It could take weeks, months or even years as they focus their attentions on others but usually they check in using some method at some point in time. My ex actually still keeps in touch with several past exes (and lovers, even one night stands) as 'friends'. Some examples of how my ex did this to me when we were not together:
Seeing me at a mutual friend’s birthday party and texting me to meet her outside despite her ignoring me for several weeks and I giving her no indication I wanted to speak to her at this engagement.
calling me up to help her get a rental car because her car broke down and she didn't have anyone else to call (2 weeks mind you after our biggest break up and her asking me to move out of our shared home, because I had nowhere else to stay she had been staying at her parents. This also happened the very day I was moving to my new place. Oh, and I later found out that she had already been sleeping with an ex lover!).
Telling a friend of mine she barely knew within 5 minutes of an online conversation she initiated that she was doing fine and had been dating a "nice" guy for two months when it had been less than two months since she broke things off, she had indicated (LIED) that she had missed out on a lot of her friends lives while we were together and had been selfish and needed to focus on building those relationships she intended to have for a long time. This was also a month after I had stopped all contact (even third party) due to legal boundaries she set up.
And yes I gave in every time and helped or replied in some fashion, that's how addicted I am!
because I had taken her broken laptop to get fixed (and paid for it of course!) and only I could get it back and after she had filed a police report for stealing it, I sent her a brief text saying it was done and when she wanted to meet in a public place to get it. She responded by calling me up a bit tipsy, to come and get her from a party she was at. I did and after some amazing sex we got together for the last time (later found out she was at the guy she was currently dating birthday party and in the previous week we had been at a mutual event and she had completely ignored me, like she never even knew who I was!)
After a 2 year 'relationship" aka addiction to my borderline and after several horrible breakups (police, suicide attempts, physical and verbal abuse, slandering, calling police, her spreading horrible rumors to my boss, parents, friends, etc) I discovered we would date for about 4 months and then break up and be off for 2-3 months (I was usually split black at this point), then we'd get back together (and people would be like WTF?!?).
In my most recent break up (2 months ago) she initiated it as a preemptive strike because I'm sure she perceived my behaviors as 'abandoning" despite our history and those things had not been as crazy this round. She did however keep a few hundred dollars worth of my stuff, despite having three separate opportunities to get everything back to me. I did get some but NOT everything. She then proceeded to completely ignore me and after I had a third party step in a following a lawyer’s advice sent her and email describing the items and worth, indicating I would file a police report if she did not comply. Well big mistake she called up the police and reported that I was harassing and stalking her. Of course she forgot to mention that we had been seeing one another recently and indicated back to when she had filed a report with the police that I had stolen her laptop (I didn't and it went nowhere), plus she did have some previous emails where I said some not so nice things from a previous breakup (crazy by proximity anyone), so she definitely had some ammo. I was given a warning that all she'd have to do is report me again for any reason and I would go to jail so I gave up. But see how this left an opening to contact me again in the future?
Funny thing is I managed to get mugged during all this so I ended up having to change my cell number but she still has my email. I waiting for the 3 month period to come and if she'll inmate contact again somehow (prolly be pissed when she realizes my # changed

). After she called the police and I ceased any contact and my attempt to get my things back she contact a random friend of mine that she barely knows and within 5 minutes was telling him that she had been dating a 'nice' (see how I'm black and he's now white) for the past two months. Well we both know it's been less than that since she broke things off! Was she trying to get a reaction from me? I think so!
All this despite her sounding logical, sincere and regretful during the breakup. I just want us to both move on she said!
Sorry for the long post!
OUCH! That knife you stabbed into my heart hurts when you twist it.