IMEI number complication - need help from someone knowledgable

[arnoldlayne42]

Okay so here's the deal summarized:

1. I bought a used Note 2 off a friend's friend.
2. After a few months I noticed that the IMEI that's displayed inside the phone is different to the one beneath the phone.
3. Didn't really pay attention to it, because it didn't affect me.

Now I lost my phone a few days ago, and the authorities are asking for the IMEI number of the phone to find it. Now here's where the problem lies. Can someone who knows about IMEI nos tell me what IMEI number my phone would be tracked by? I don't have the IMEI which was beneath my phone's battery so if that's the one which would matter, then I'm screwed.

Some more information, I flashed 3-4 ROMs on it including stock and AOSP but still the IMEI inside the phone remained the same. Plus, on my PC, in Google->Accounts->Dashboard->Devices, I can see the N7100 with it's IMEI same as the one which displays inside the phone.