Hey, thanks for the reply and trying to help me out!
Yes, I continued to look for new Pikachus after that one disappeared! But this specific one was level 4 and was caught before them. I'm not sure if you can check this kind of info, but the Pikachu that disappeared was caught after the Diglet, and before the Poliwag that show up on my PC and I'm sure it was caught because the Caught Pokemon Preview was activated and its stats were shown to me.
I'm sorry to insist on this, but its IVs were so good that my heart just can't let it go hehe!