This is probably a very common question. However I'd like some opinions outside of mine to help me understand the why.

Some background:
My ex-gf and I broke up about 2 years ago. We had a pretty bumpy relationship with many lows and few ups, probably due to our (mostly mine)inexperience and immaturity. I was really jealous and made obvious attempts at controlling and changing her lifestyle, which didn't work and caused a lot of friction between us. She did the same to me and tried to change my lifestyle, personality, etc, she succeeded at it, but that's irrelevant. The point is we were very similar beings but we weren't really compatible, I later realized that.

I can't really tell who broke up with who. At the heat of the moment I got dumped, and that seemed to be the end of it. However, we both broke the no-contact rule within a week or two after the incident. I'm not sure how to put this nicely, but I got the impression (from our conversation) that she did not want to date me anymore, but did not want to lose contact with me, and wanted to see me kind of secretly, on the side or so I interpreted. I did not (still haven't 2 years later) delved deeply into the why she still wanted to see me, but I already had enough with this girl so I decided to break all ties, out of the blue, with no notice.
And so I completely stopped talking to her. Proceeded with the no contact method, got myself very busy with work, school, and hobbies. In a relatively short amount of time I stopped feeling the post-break-up-sadness most of us have, I was a changed man. Eventually I started dating again.
Over the last two years she has sent me several messages through Facebook. The content of such messages evolved from asking me why I hadn't talked to her in so and so long, to explaining how she dreamed and thought of me to wondering how I am doing. The attitude of said messages also changed from "i don't really care" to "i wish" over time, no evidence of neediness at first sight.
I have never replied to any of the messages. Me and my current girlfriend have an unwritten rule about not actively communicating with our ex's, since we both have had bad experiences (cheating, sneaking out with them, lies etc) with our ex's previous partners intruding in our previous relationships. I understand this may sound confusing, but we both basically want nothing to do with our previous partners.

So I guess my questions are:

1- Why would my ex try to communicate with me behind her (multiple) bf's back?

I understand people think about their previous partners occasionally, even dream about them, get curious etc. I think it's normal. However in my case it does not make me inform her about it.

2- Do you think there is an underlying message in between the lines?

3- Why not just call me?

4- Should I reply to these messages to satisfy her curiosity?

5- In general why do some girls try to contact the ex they left?

It's a long post, but I feel the background info is relevant

1. Probably because you suddenly applied NC and stuck to it, which, left her wondering. Her recent boyfriends may not have been so convicted.
2. Nah, it's likely a test to see how much influence she has on you. By not responding, you send the message that she has none.
3. Because sending a Facebook message is easy and because she's not actually curious about your wellbeing, she just wants to know if she still has influence. Given this reason, calling would seem very selfish and wrong, even to her.
4. I wouldn't if I were you. You're in a position of power, maintain that.
5. Very few exes are actually friends and do call because they care. The majority contact exes for the reason I already pointed out.

1] Because she's unhappy with her own life at the moment and is pondering the "what if?"s.

2] No, just curiousity.

3] Because it's a lot easier to Facebook message. Calling takes more courage!

4] No, not if it's just to satisfy her curiousity.

5] It's not limited to girls. For example: my ex contacted me (on Facebook) after almost 10 years. At the time, he was going through a rough patch in his marriage and was viewing our relationship through rose-tinted specs! For the record though, we kept it friendly and now remain on friendly terms, which is nice. No awkwardness... but almost 10 years HAD passed! ;)

This is probably a very common question. However I'd like some opinions outside of mine to help me understand the why.

So I guess my questions are:

1- Why would my ex try to communicate with me behind her (multiple) bf's back?

I understand people think about their previous partners occasionally, even dream about them, get curious etc. I think it's normal. However in my case it does not make me inform her about it.

She must not be happy with the guys she has been with, and maybe thats why she has had a few, and still wondering if she can have you again. Maybe she just needs the attention of a friendly voice, maybe its curiosity. Who can really know whats on her mind.

2- Do you think there is an underlying message in between the lines?
Any guess a stranger makes is just a guess, but she IS wanting to get your attention, or get you thinking of her, or at least get you curious as to what she may be up to, and it worked.

3- Why not just call me?
Thats too risky, and facebook is easy.

4- Should I reply to these messages to satisfy her curiosity?
Hell NO!!!! That would betray the agreement between you and your current girl! Not worth the risk.

5- In general why do some girls try to contact the ex they left?
I think its mostly because they can, and maybe their lives are not in a good place, or its easy nowadays with Facebook, and other networking social sites. Could she be wanting to cause you problems? Make your girl jealous? Anything can be the reason.

The bottom line is to block her, and end all the speculation, assumptions, and curiosity. Then NO drama. Her motives, curiosity, or whatever else is in her head, is no longer a concern. Its probably a dumb fishing expedition on her part, since you did disappear from her radar, and that was a good move.